While it might not have been comfortable I had two users separately logged in to a V30 machine (8086 clone) back in the day, one over serial console. That was with Minix 1.5. OS-9 on a CoCo could do a similar trick on even more limited hardware (6809 processor). The problem was with DOS's design.

(Video of someone doing two users on a CoCo: https://www.youtube.com/watch?v=deBPEPPIdwk)