I was thinking about the s100computers.com web site and looked up and remembered that I had another S-100 processor card on my shelf a bit less common than the Cromemco ZPU.
This one has two processors, one being the standard-fare Z-80 for maximum software compatibility and the Motorola 68000 16-bit processor.
I never used this card myself but when it was offered to me, I took it because of the uniqueness and the possibilities it would have offered in “the old days.”
There is more information about the card including the manual for it on the s100computers.com web site.
Link: Cromemco ZPU on S100Computers.com