mc cross fader ?

there might be many ways(for example, could also mc.unpack~, crossfade, then repack using mc.pack~...), but i'd use 'mcs' version of matrix~ like this:

(if on the other hand, you meant crossfade multiple mc signals in tact, then just the regular 'mc' version of matrix~ would do it, too)

Is it possible to control the amount rather than either 0 or 1 ?

ya, that's just in my example, see the helpfile for matrix~... or in my example, you can delete the metro, and just mouse-and-drag on the matrixctrl, it sends floating point between 0 and 1 too(copied from the helpfile)