Gemara

2a (Mishnah): Partners who split a garden make a wall in the middle.

Suggestion: The wall is obligatory, due to Hezek Re'iyah (invasion of privacy).

Rejection: No, it is required in a garden for a different reason:

(R. Aba): One may not look at another's field when it is laden with crops (due to Ayin ha'Ra (envy), which can ruin one's good fortune).

4a (Rava): Normally, a valley is considered a place where people don't make a wall, so one neighbor cannot force the other.

5a (Mishnah): If the wall of a Chatzer fell, they must rebuild it to four Amos.

Bava Metzia 30a (Beraisa): If one found a sheet, he may not spread it on a bed, nor on a peg for his needs. If guests came, it is forbidden for his needs and its sake together.

This is lest it be lost due to Ayin ha'Ra, or lest it be stolen.

Pesachim 50b (Beraisa): If one seeks to profit through sticks or bottles he will never be blessed through them. Since they have a big volume (it looks like the owner has much merchandise), Ayin Ra'ah prevails over them.

Rishonim

Rif and Rosh (Bava Metzia 16a and 2:20): One may not spread out a sheet (an Aveidah) when guests came, due to Ayin ha'Ra, or lest it be stolen.

Rambam (Hilchos Sechirus 12:1): "Ki Savo b'Kamas Re'echa" refers to a worker. If not, who permitted him to come to another's vineyard or crops?!

R. Matisyahu Strason (Bava Basra 2b DH d'Omar): Perhaps the Rambam hints to the law that one may not look at another's field laden with crops. Alternatively, the Rambam normally omits laws that depend on Ayin ha'Ra, Ru'ach Ra'ah, Shedim...

Rashi (Bava Basra 2a DH Asur): One may not look at another's crops lest he harm them through Ayin ha'Ra.

Rambam (Hilchos Shechenim 2:16): Partners who split a garden can force each other to make a wall 10 Tefachim tall.

Question (Chachmei Lunil): The Gemara in Bava Basra connotes that there is more Hezek Re'iyah in a garden than in a Chatzer. If so, the wall should be four Amos tall, like for a Chatzer!

Teshuvas ha'Rambam (brought in Migdal Oz): Hezek Re'iyah is great damage. Concern for Ayin ha'Ra is only Midas Chasidus. The Gemara said that it is the reason for a wall in a garden merely to repel the proof. The Halachah is not concerned for Ayin ha'Ra. Ayin ha'Ra is greatest in a valley (there are more crops), and we do not require a wall there!

Lechem Mishneh: The question and answer both require a wall in a garden! The one who answered that a garden wall is due to Ayin ha'Ra agrees that the Mishnah exempts a valley! Tosfos says that there is more Ayin ha'Ra in a garden, in which different species grow throughout the year. Ayin ha'Ra in a valley is only one month a year (when the crops are grown). It seems that the Rambam holds that the question and answer are both unlike the Halachah, for we rule like Rav Nachman in monetary laws; he says that Hezek Re'iyah does not apply to a roof, and all the more so to a garden.

Gra (CM 158:3): What was the question from a garden? Perhaps it refers to when they agreed to make a wall! The Rambam holds that we conclude that the wall is when one sold half of his garden. It is not for Hezek Re'iyah, rather, so it will be clear that one who crosses the border intends to steal. It cannot apply to partners, for they did not suspect each other until dividing!

Poskim

Shulchan Aruch (CM 378:5): If looking will damage someone, it is forbidden. Therefore, one may not stand by another's field laden with crops.

SMA (4): The Tur and Shulchan Aruch discussed specifically damage through Ayin ha'Ra, not Hezek Re'iyah (CM 154:3). They discuss only Ayin ha'Ra here. 'Therefore' connotes that it is forbidden only because it itself damages. Other Hezek Re'iyah does not itself damage, it only causes damage, i.e. he cannot do his needs became he can be seen. However, all the more so Hezek Re'iyah is forbidden, for one opinion in the Gemara forbids Ayin ha'Ra only due to Midas Chasidus.

Hagahah (in Friedman Shulchan Aruch): Teshuvas ha'Rambam says so. No opinion in the Gemara explicitly says so.

Shulchan Aruch (EH 62:3): If there are two Chasanim together, we make one Birkas Chasanim.

Rema: Some say that we do not bless two Chasanim together, due to Ayin ha'Ra. The custom is to make a separate Chupah for each. However, we may make a meal for many Chasanim together, and bless only once.

Source: Hagahos Maimoniyos (Hilchos Ishus 10:3), brought in Darchei Moshe (3).

Drishah (6): Similarly, due to this opinion we do not bless on and circumcise two babies together, due to Ayin ha'Ra. This is even though one could distinguish: we could bless once and circumcise in two places in the Beis ha'Keneses, for then Ayin ha'Ra does not apply so much.

Shulchan Aruch (YD 265:5): If one has two babies to circumcise, he blesses once for both of them.

Taz (11): Teshuvas ha'Rosh (26:4) says so, and also regarding two Chasanim. Why did the Rema disagree only regarding Chasanim? It seems that to make one Berachah for two Chasanim, since they are obligated, they must be together at the Berachah, and Ayin ha'Ra applies. The babies need not be near the Berachah, so Ayin ha'Ra is not a problem.

Pischei Teshuvah (YD 116:6): Adnei Paz (25, 34) permits calling brothers the same name. However, it is proper not to, due to Ayin ha'Ra.