I'm an electrical engineer, working in aeronautics, and I have a fair background in machining, power electronics and CAD from college. Nevertheless I'm not an expert on rotating machines, so I'll give you as much info as possible... and I welcome questions and comments.
The driver I use is a made-in-China CW250-AC, which can be picked-up on eBay under a number of different names. It's one of those integrated microstepping drivers with screw-on wire terminals and a big heat-sink. Very practical and works surprisingly well.
I've set it up for 1/5th microstepping and its maximum allowed current, which is 5 amps. It's driven by a PC through a crude break-out (the CW250-AC has input optocouplers, no need to overdo it)
Power comes from a 150W 24V switch-mode PSU, way overkill but I don't have anything smaller on hand.
The motors are two identical 85BYGH450C-03A, specs are :
- NEMA 34, 150mm body (what I call big-ass : they weigh 5kg each !)
- 6.3 N.m torque at their rated current of 4 Amps.
- 200 steps/turn
- insanely low coil resitance and inductance
I first tested each motor separately, then both in parallel.
If you intend to try this, let me advise you to check very carefully your connections with an ohm-meter : you really don't want to have one motor correctly wired and not the other.
One fun test : if your steppers are correctly wired in parallel, unplug the controler and spin one shaft by hand : the other motor's shaft should make the exact same move. If it moves in the reverse direction... you've made a deadly mistake :-)
On to the results...