You are correct that Russia and China will never really trust each other.
But they share an interest in destroying the "rules based international order" (code for US hegemony).
Germany and Japan never really had much in common but were allies in WWII.

China's population (fertility 1.1) is declining much faster than Russia's (fertility 1.5). China has huge empty cities. It does not need more space.
China wants to completely control every aspect of its people's lives with technology.

It is much more dangerous to freedom than Russia.