Using a foreign CPU architecture for a co-processor was a very common practice in the 90s, even early 2000s; e.g. the GameBoy Advance has a regular ARM CPU as it's brain, but it also hosts an entire GBC on board (including a custom Z80/8080 clone/hybrid) - mostly for backward compatibility, but it performs other regular duties (such as sound) even in "GBA mode".

GBA is a fascinating system, the main CPU is powerful enough to do some serious work - some games even had pretty decent 3D (check out Colin McRae Rally 2.0); all while remaining simple enough architecturally that you could mostly keep the whole thing mapped out in your head. You get to learn about many modern concepts such as instruction pipelining, caches, etc but it's not as complex as to overwhelm you, and you still get to play with funky quirky stuff like the sound chip, that throws you back into the 1980s. You can also still get them pretty cheap, I paid ~€120 for a GBA SP (AGS-101) with case, dongles & a flash cart, so I can trivially run homebrew stuff on real hardware.

https://en.wikipedia.org/wiki/Game_Boy_Advance; https://duckduckgo.com/?t=ffab&q=colin+mcrae+rally+gba&iax=i...; https://gbadev.org ; https://docs.rs/gba/latest/gba/