Housing different species together isn't a good idea. Tortoises can carry bacteria in their systems that is harmless to them, but may be fatal to other species.
Two Russians MIGHT be able to share an enclosure, but there are a few things to look out for. Two males are likely to fight. A male and a female may work, as long as the male doesn't continually harass the female with mating attempts. This can stress her out and possibly be detrimental to her health. The enclosure should be large enough, and there should be plenty of hiding places for her to go into.
I've housed a male and female Russian tortoise together, and they were fine. However, a friend of mine got a female to keep his male company and had problems with him constantly going after her. So you never know for sure if a particular combination is going to get along. Be prepared to provide separate enclosures if it doesn't work out.
Incidentally, it's recommended that any new tortoise be kept separate from the other for at least 3 months to make sure it's healthy without any communicable diseases. During this time, the new addition should be checked over by a vet, including a fecal check for parasites.