What about PDP-10 systems? Those could be SMP systems.
I know of one
person who ran a 3xKI10 system with TOPS-10. True SMP...
And then we have the PDP-11/74 (up to 4 CPUs), but they are probably
even harder to find than KI10 systems.
For non-SMP systems, there's the PDP11/10S. There's a link on one of the
CPU boards in that machine that disabled the bus arbiter and makes it run
as a slave processor on the Unibus of some other PDP11. I've never heard
of anyone doing it, but it's in the printset...
-tony