The reason I have chosen to use two motors is because I have a belt drive machine with one motor and it suffers belt issues and does not go fast enough, along with the fact that there are many many commercial build tables with slaved motors and AFIK no hardware or software fitted to check for a failed move on one motor i.e. racking the gantry.

With steppers, its a moot point to a degree as they will not do any (or much) harm if stalled, a servo on the other hand will happily destroy itself and/or its driver when jammed. Gantry damage is possible with both systems i guess but to different degrees.

My motors have dual shafts so I guess I could install slotted opto-discs and detectors on each motor and a small logic board that trips the E-Stop circuit if one side does not move - could be an interesting project one day.

Power-up IS however one issue I am looking at, I know my steppers jump into lock when powered up, but what are the chances of BOTH jumping into the same lock position ?? Zero, 50/50 ????

That is the main point I think