OK tomorrow I'm gonna try some stuff out and come back to annoy you people again when it probably doesn't work.
I just can't understand why motor 2's motion would cause motor 1 to stall. Motor 2 is the one using a lot of torque for rapid back and forth motion, motor 1 just spins up with 4000 step acceleration (in accel stepper) to 800rpms and somehow seems to affected along the way. Without load too.
e: while I'm posting, do you have any suggestions for power supply & driver types (maybe motor size too?) that'd definitely give the torque needed for rapid back-and-forth movement with 1/16 or 1/32 microstepping on a 4-start lead screw? I was under the impression that what I had would do it.