They don't have anything like a cordial neighborly relationship. Nor have they had during the long history of that long, long Asian border.

Even at the height of the Cold War, they were political rivals in the Communist camp. Just one example, lest we forget: Ho Chi Minh played them against each other while fighting the US. His biggest fear was not the US, but that China would then overrun Vietnam as it had in imperial days. So he courted Soviet involvement, which he received, as insurance against Chinese hegemony.

Joint military manoevres are not only a good way to sell your wares, as Tom points out above. They're a good way to see a little of what your "partner" has up his sleeve.

China and Russia are not natural or traditional allies; they merely have several common interests.