[Unless China and Russia scheme together.]
No benefit for Russia, unless its long-term goal is to become a Chinese province. During WW2, a big reason for the Japanese not attacking* Russia and tying up Russian troops in the Far East, thereby preventing them from reinforcing shattered Russian forces in Europe, was likely because they anticipated that a Germany that had conquered Russia would turn its attention to Japanese territory. Historically, most alliances have been opportunistic, with alliance members gobbling each other up, given the chance.
* Khalkin Gol, a skirmish between Russia and Japan was technically a Russian victory. In reality, the Russians lost more men and equipment despite being more numerous and better-armed.
https://en.m.wikipedia.org/wiki/Battles_of_Khalkhin_Gol#Aftermath