1)
Why are Sichon and Og mentioned separately?
1.
Rashi (17, from Yalkut Mechiri): Each by himself is equal to the 31 kings.
2.
Seforno (19-20): Sichon was a trained warrior, and "k'Govah Arazim Gavho" (Amos 2:9); Og was a giant 1 , and his land was greatly fortified.
1
I.e. even bigger than Sichon. Seforno implies that Bashan was more fortified than Sichon's land;. Rashi (Bamidbar 21:23) implies that Sichon's city Cheshbon was the most fortified. (PF)
