When you come to reverse the gantries direction of travel you have to stop it then start it again.

How quickly you can do this depends on...

The weight of the gantry
The weight of any rotating components
The feed rate
The power of the motor

If made friction free a tiny motor could drive a heavy gantry but the accelerations/decellerations would be disappointing.

It won't be friction free so the motor also has to overcome the rolling resistance.

Two motors on one axis is asking for problems, suggest you run a light weight aluminium tube side to side and use it as a drive shaft.

Don't blow all your budget on monster steppers and high voltage drivers, you could end up with a 4.2 litre milk float.