Correction to previous post: I said that, in Single mode, the MODX keys can't play a sound that is different from what is being triggered externally, but it can, the limitation being that the it can only be a single-part sound. (It is done by selecting a part from the MODX Performance screen, since selecting a Part always makes that Part--and only that Part-- playable from the internal keys.)
So getting back to the OP's scenario, I believe the options are:
* Single Mode: Up to 8 MODX Parts played from the MP11, with 1 other Part playable from the MODX itself (or the MODX can also merely be a duplicate of the up-to-8 parts that you're playing from the MP11)
* Multiple Mode: Up to 4 MODX Parts played from the MP11 (since MP11 has a built-in 4-zone MIDI controller function), with up to 8 other Parts playable from the MODX itself.
Though remember that as soon as you've assigned more than 4 Parts to a Performance (regardless of whether they are being played by one keyboard, the other, or some combination), you will lose the seamless sound switching.