If anybody can get to the old usenet comp.sys.amiga discussions of this topic from > 15 years ago, they would be very informative. CBM itself, when in business, was looking into this when there were actually many full time people working on the OS, and Dave Haynie was looking into multi-CPU experimental hardware. He may have actually built a dual-68K board, I can't remember now. The basic problem was, as always, it would break too much existing software; of course, that was when there was commercial software still being written!
The 'original' Los Gatos Amiga crew were very much aware of the problems related to the 1983-1985 design decisions, and tried to apply what they learned in their next design, the 3DO operating system. I don't know if they made it SMP friendly or not, but it suffered the same ultimate fate as Amiga.
