I have worked with many architectures and, yes, that is not "a problem". However, one thing I have found is that while I am an ardent proponent of "the best chip for the job" is that in a small multiprocessor application you do gain some advantages by keeping the same architecture on all nodes. Thus if the issue is a 'satelite' to a '51 I would 10 times over prefer that 'satelite' to be a '51. Now if the 'master' is a behemot, you will, of course, use a different architecture on the 'satelites', but in such cases the development effort is already high.

Basically you have two situations
a) the "I need another UART" or something similarily simple
b) This we will do with one master and "47" satelite processors

in the case a) I would stay within one architecture since that allow an easy transfer from satelite to mother. In the case b) such transfers will be irrelevant and thus no architecture preference (except "the best chip for the job")apply.

Erik