Let's make it clear so we'll stop wasting time arguing on what's possible or not (lol):
iMac Pro needs RDNA2.0. RDNA2.0 is scheduled for release in Q4 2020. Plus, it will have hardware ray tracing, which is a nice marketing thing for Apple "you are going to be able to do real time ray tracing * insert audience wow here with a cute scripted demo from people knowing their text by heart *".
So no iMac Pro until Q4.
iMac will have up to NAVI 10 XT, which is mid range GPU, and this is fine for iMac since iMac Pro will have the top tier GPUs with more powerful Cascade Lake Xeon CPUs.
That's it ! It's clear, simple, defined. Both don't overlap each other, just like now.