is there a way to use the M2PC console as a fader wing with the NX-4 console?
No, they would simply mirror each other.
You could use a laptop in a Master/Slave network setup and turn off the bank selection which will give you a second set of faders. I have used on several film sets when running a network setup and two operators for a live record. My main cuelist has a macro that triggers the master console to go to bank 2 which I have my cuelists I use during the record.
I also run two M-Series submaster wings on the same show and I highly recommend them if you can find a second hand one somewhere. I would love to see them come back under the Onyx brand “wink wink nudge”. They are great for live busking .