Gemara

Question: May a Nidah sleep [in the same bed] with her husband, when each is in his or her own garment?

Answer (Rav Yosef -- Mishnah - Beis Shamai): Fowl may be put on the same table as cheese, but one may not eat them together;

Beis Hillel say, they may not be put on the same table nor eaten together. (Likewise, we decree about a Nidah.)

Support (Beraisa): "...V'Es Eshes Re'ehu Lo Timei v'El Ishah Nidah Lo Yikrav" equates a Nidah to another man's wife:

Just like one may not sleep [in the same bed] with another man's wife, even if both are clothed, the same applies to Ishto Nidah.

Rav Yosef argues with R. Pedas;

(R. Pedas): The Torah ("Lo Sikrevu Legalos Ervah") forbids only actual relations with Ervah.

(Tana d'vei Eliyahu): A case occurred in which a Talmid who had learned and served Chachamim greatly died very young. His widow was asking why his Torah did not earn him long life.

Eliyahu: How did he act with you when you were counting seven clean days?

The widow: He would eat and drink with me and sleep [in the same bed] without clothes [on either of us], but he never even considered relations!

Eliyahu: Blessed is Hash-m who killed him [for transgressing b'Mezid]! It says "v'El Ishah b'Nidas Tum'asah Lo Sikrav"!

(Rav Dimi): The Talmid had one [wide] bed. (He would not contact her. Therefore he was lenient to sleep with her.)

(Rav Yitzchak bar Yosef): She used to wear a garment over her Ervah. (Therefore he was lenient.)

Eruvin 63b (Rav Berona): It says about those who sleep in a Kilah (canopy, or small room) where a couple dwell "Neshei Ami Tegarshun mi'Beis Ta'anugeha (they separate women from their husbands)".

(Rav Yosef): This is even if she is Nidah.

Rishonim

Rambam (Hilchos Isurei Bi'ah 11:19): A Nidah may not set her husband's bed in front of him.

Magid Mishneh: The Rashba brings from Rav Hai Gaon that he may not sleep on her bed, even if she is not there. This is reasonable, for even setting his bed in front of him is forbidden, and sleeping on her bed is more likely to lead to Aveirah. The Ramban agrees.

Rosh (Nidah 10:2, citing the Ra'avad): Rav Hai Gaon says that a Kalah who has her first Bi'ah and sees blood finishes the Bi'ah, then sits on the side like a Nidah, lest they come to Aveirah. It seems that he forbids sitting on a Nidah's bed even when she is not there. This is only for a Vadai Nidah. One may sit on the bed with his Kalah after separating, even on the same sheet.

Beis Yosef (YD 195 DH u'Mah she'Chasav v'Lo): Even though the Ra'avad forbids only sleeping on her bed, the Tur understands that one may not sit on it, for also this arouses Hirhur (lustful thoughts), which can lead to Aveirah.

Mordechai (Shabbos 1:238): Tzafnas Pane'ach writes in the name of Rashi that one may not sit on a long chair with Ishto Nidah. Maharam says that one must be careful that if there are two beds, and the legs of one touch the other, he and Ishto Nidah may not sit on the beds unless there is an interruption in between.

Terumas ha'Deshen (251): It seems that man may sit with Ishto Nidah in a wagon to go to another city if they will not touch at all, e.g. he is in front and she is in back. The Agudah permits. Even though the Mordechai brought in the name of Rebbi that one may not sit on a bench with Ishto Nidah, and this is similar, there is a reason to distinguish. In Hilchos Nidah it is written that this is a mere stringency. We can say that this is only to sit on a bench together in a way that shows affection, the way lovers gather together. It is normal for strangers to hire a wagon together to go to another city. This does not show affection at all. Even if he and his wife are alone in the wagon, and it is their wagon, this is not the way of affection. I heard that a Gadol permitted sitting on a bench attached to the walls of the house, like is often done in winter or summer rooms. He says that it is forbidden only when it is detached. However, it is not clear to permit going with Ishto Nidah in a wagon for a pleasure trip to gardens and vineyards.

Tashbatz (3:42): If a bed is fixed in the house with plaster, or even in beams fixed in the wall, and it is a long bed and her bed is at the foot of his bed, not near her feet, surely it is permitted. This is just like two adjacent beds in the house! However, if they face each other it is forbidden, for it leads to Aveirah, even if each is on a separate sheet, unless there is a great distance between them, or a Mechitzah (barrier) sheet separates them. We say in Sanhedrin (37a) that if there is even a fence of roses between a man and Ishto Nidah, they will not sin.

Poskim

Shulchan Aruch (YD 195:5): One may not sit on his wife's special bed, when she is Nidah, even not in front of her.

Bach (5): The Beis Yosef's text of the Tur forbids sitting. This is astounding. What is the source for this? The Rosh forbids only sleeping! In Sof Siman 193, the Tur forbids sleeping. The Beis Yosef's text is mistaken. The text should say 'he may not sleep on her bed.' Other texts forbid lying. This is the correct text. Bedek ha'Bayis (the Ro'oh) forbids only lying and sleeping without clothes. He permits sitting clothed on her bed in her absence.

Taz (6): It seems that mere sitting is forbidden even without lying. Sitting also leads to Hirhur, which lead to Aveirah. The Bach holds like the texts of the Tur that forbid [only] lying together. He supported this from Siman 193, in which the Tur and Shulchan Aruch forbid sleeping. I disagree. There, they teach that regarding a Kalah, even sleeping is permitted! Surely, sitting leads to Hirhur. The Rema brings from the Mordechai that they may not sit on a bench together. All the more so, sitting on her bed is forbidden. It leads to Hirhur more. All the more so, she may not sleep in his bed. There is more Hirhur when she lies down and gets up. She may sit on his bed, for it does not lead to Aveirah.

Rema: One may not sit with her on a long bench that shakes and is not connected to the wall. Some permit when another person is in between.

Source: Bedek ha'Bayis brings this from Orchos Chayim.

Beis Yosef (DH v'Chasuv Od): This stringency not to sit on a bench together, I heard that Ashkenazim are stringent for it, but Sefardim are not.

Rema (ibid.): Similarly, one may not go with Ishto Nidah in a wagon or boat for a pleasure trip to gardens, vineyards and similar things. If he goes from one city to another for a need, it is permitted even if they are alone, as long as they sit in a way that they will not touch.

Beis Yosef (DH u'Mah she'Chasav v'Lo): The Rashba brings from the Ra'avad that Rav Hai Gaon forbids sleeping on the bed of Ishto Nidah, even if she is not there, lest this lead to Aveirah, like the Magid Mishneh wrote.

Pischei Teshuvah (9): Surely, it is permitted if she is not in the city.

Shulchan Aruch (6): He may not sleep with her in a bed, even if each of them is clothed and they do not touch.

Rema: This is even if they are on separate sheets, and even if they sleep on separate beds, and the beds touch.

Beis Yosef (DH Kasav ha'Mordechai): R. Yerucham, who brings from Rashbatz that if the bed is wide and they are far from each other, surely they may sleep clothed on it, for Chazal forbade only closeness. If each is on a separate sheet, it is merely seclusion, and it is permitted. The law of a bench and Maharam's law connote unlike R. Yerucham.

Maharam Alashkar (91, brought in Shach 11): In a bed of wood or of Binyan, each may sleep on his or her bed within the curtain. We learn from Eruvin 63b. "Kilah" is from the root Kela'im (curtains). It is the tent or curtain in which the man and his wife sleep.

Machatzis ha'Shekel: 'A bed of wood or of Binyan' is like a small room with room for only two beds. Kilah (in Eruvin) is such a room. It is improper for a stranger to sleep there with a couple, even when she is Nidah, for they are ashamed to speak in front of others.

Mekor Chayim (33, cited in Pischei Teshuvah 11): The Mordechai connotes that this is only if the feet of the beds touch each other, for then he and she face each other and can see each other and come to Aveirah. If the head of one bed touches the foot of the other, and all the more so if the heads of the beds touch, one cannot see the other's face, so it is permitted. People are stringent like the Rema said, without distinction. Any separation suffices.

Pischei Teshuvah (11): Chachmas Adam (116:5) says that if the beds are attached to the wall, perhaps it is permitted. I say that the Rema discussed only their beds, which did not have boards sticking up. Then, when the beds touched, they looked like one long bed. Our beds are like a box enclosed by boards above, so it is permitted. Ikarei Dinim (21:46) says that even though Maharam Alashkar permits sitting with Ishto Nidah in a Kilah when each has his or her own blanket, it is proper to be stringent unless a hanging sheet divides the beds, so they cannot see each other. It is clear from Tashbatz that without this, it is forbidden.