I find the MKS-30 and JX-3P *very* different in sound character - so much so that I wouldn't even consider either of them as an expansion of the other's polyphony, that would just sound weird :-)
And to stack them, it suffices to set them both to the same MIDI channel.
Two 3Ps or two MKS-30s would of course work, but is anyone really going to buy two, including the upgrades?