I believe you could have 7 steppers hooked to one drive. If they are not loaded to beyond the amount of current the drive will handle. The G201 will handle 7Amps. If each motor on each ball screw only has a max of 3.5 amps then you could safely use two motors. If you verified that the load on each motor in your system would not draw any more than 1 amp you could put 7 of them on one drive. There are a lot of factors that put a load on the motor. The size of the ball screw, its resitance to turning... which is small in most cases. The type of bearing of your screw and carriage. The weight of the carriage.
The negative to using two motors would be there power. A 4.5 amp single motor that has 400 oz inch torque would be my choice over two motors that only have 3.5 amp ratings at say 150 oz inch.
Contact Marice at Gecko drive for the best tech. support on this issue... He is always glad to help.
And real world experience is also the best way to learn. Build your carriage and try out both senarios. Not hooking up the motors permanently. Most manufacturers will except returns with only a small discount when returned.
One motor is simple, the simple solution is usually the best. The other issue with two motors is How will you recover or resychronize them if one side gets out of wake... You would need permanent marks that you could see... so you could put them by hand without power back in sync... With one motor this is not an issue.
Go one motor... it will be better... although you could do the other. The other will be more hassle, most likely less power, and cost more in the end to mount and wire.
Let me know how it works.