Silly idea: could they put a M-series processor connected to a Xeon as a coprocessor to get the “benefits” of both wrt. adding their NPU stuff? Would that even make any sense?
The two tricky things I'd anticipate are:
What does it look like to hardware? Different CPUs speak different protocols to maintain a cache coherent view of memory. So they couldn't easily share memory in a completely natural way - but GPUs manage to share memory between host and GPU, so it can be done (just not necessarily as conveniently to software).
What does it look like to software? Lots of weird options here. Most convenient for software compatibility is it the Intel is also somehow running MacOS (or appears to be). I guess you could do that - it'd be like having a two node cluster in one machine. But that's not as convenient as being able to distribute computation between compatible CPUs.
None of that is a complete technical blocker to a company that controls their stack, I'd just guess it would make it too inconvenient / expensive for them to do.
That said - as an intern, years ago, I once had access to a piece of prototype hardware that had an x86 chip in one motherboard CPU socket and a non-x86 chip in the other (with various adaptation hardware to allow them to play nice). I think that was more a proof of concept than anything else, not necessarily viable without other hardware changes. Interesting nonetheless.
I saw a lot of new-in-box examples cheap on eBay prior to Sun's implosion, so I suspect they weren't particularly popular and were primarily targeted at a few important customers that had some box to tick.
Eg: https://everymac.com/systems/apple/mac_quadra/specs/mac_quad...