On mine I have them on one platform. The theory has always seemed to be to keep belt driven TTs and their motors on the same one, but I for one don't think it is absolutely necessary. If they are separate, you could experience small changes in belt tension based on either up and down movements or, worse, side-to-side movements but for most of the very small movements (which are the ones which matter most to the TT), I expect they would not be problematic. Never having tested the theory, I cannot speak from experience, and someone with experience, or abetter grasp of the physics involved, might feel differently.
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You probably do not want to mount the motor on a separate platform because cogging of the motor will excite resonances of the damped suspension system. This will occur even with a Halcyonics or Minus-k platform. I have a a VPI TNT turntable supported on a Minus-k BM-8 isolator. The turntable's outboard motor rests on the BM-8 platform supported by a granite tile on top of three sorbothane hemispheres. There are many possible configurations of sorbothane components that can used for isolation; consult the downloadable program on the sorbothane website, which calculates the resonant frequency of a sorbothane part as a function of supported mass, shape, size, elastic modulus (durometer rating), etc. You can optimize your setup by reducing the resonant frequency of your motor support as far below the lowest frequency of the motor as possible.
With this setup wow and flutter measure in the 0.02 to 0.03% range. Speed stability (SDS stabilized) is in the 0.1% range.