No, you should not mix them. Sometimes two of the same kind of torts can live together, but you have to be prepared to have them live separately if they are not getting along.
They are solitary creatures and do not need companions.
There are those of us who will answer "yes" to that question, and those of us who will say, "No... definitely NOT!!!"
So you have to listen to both sides and make your own decision.
In my opinion, tortoises from different continents, who would never come into contact with each other in the wild, should never be put together in captivity.
Tortoises (all animals, really) have little micro-organisms (pathogens) in their systems that each type of tortoise has evolved to live in harmony with. When you add a tortoise with different pathogens, you may end up making your tortoises sick or even dead.
Now that we're getting into seeing more and more captive bred tortoises in the market-place, this won't be such a big concern. But you should never, ever mix a wild caught Russian tortoise with any other kind (in my opinion).
If you got two hatchlings of similar species, raised them together and they miraculously rarely fought each other, that would be fine.
But since Russians are so often wild-caught and come with their own internal ecosystem of bacteria, I'd never mix them with another species. They're hard to mix with their own kind!
You're putting human feelings and emotions on a tortoise. They don't have the same feelings that you do. A tortoise is perfectly happy living alone. That way he doesn't have to share the food or compete for the best hiding place. He's totally happy alone. If you DO add another Russian to your habitat, you're going to see lots of fighting. Your existing male Russian will fight a female for her to stand still so he can breed her. And he will do this constantly, every time he finds her. If you add another male Russian to your existing male Russian, you will see terrible fighting because he wants the interloper out of his territory.
In my opinion, the only time adding more Russians to a group is when they live outside in a very large pen with many, many hiding places and visual barriers.