Could be 2 females and the mounting is an aggressive dominant behavior.
Could be a male mounting a female.
Could be 2 males showing aggression with mounting.
Show us some photos of their underside and tail area and we might be able to tell you.
Regardless, your 2 Russians should be split up. Let them rule their own separate Kingdoms. We can elaborate on the "why" in more specific details once we properly identify the genders.
Pairs of Russian tortoises seldom work out. Two males will fight. Two females will fight. A male and female and the male will certainly pester the female to death for breeding.
Chances are you DO have a couple females, but the more dominant one is telling the submissive one to get out!
This is the only nasty well could be considered nasty behaviour between the two, they are very friendly, share food literally, sleep next to each other, rub faces (slowly and sweetly not aggressive) no pushing, biting, or anything... I would have split them up if I saw any but when I remove one from the enclosed the other goes around looking for them and becomes lethargic and refuses to eat or move much as though sad? This improves straight away once the other tortoise has returned... Maybe my two are strange but nature can be defied and they clearly are friends. I will upload some pics when they are awake, I hate disturbing them when asleep
You are imposing human emotions upon reptiles. They are not friends. You just don't know what tortoise aggression looks like. Sitting face to face IS aggressive. Your incorrect notions of sweetness are not going to keep these tortoises healthy for much longer if you continue forcing them to live this way. Do you think we are saying this for fun? Do you think we are just making this up to give you a hard time? We are telling you this because we have seen it many times over many years and we KNOW this is a problem. Forgive me for sounding harsh and hurting your feelings, but you don't seem to be getting the message, and your tortoises health and well being mean more to me than your emotional state.