Clamp one side so that it can't move, then push very hard on the other side. I wouldn't be surprised if it moves at least 1 mm.

Another thing to consider, is that when you apply power to the steppers, that they'll usually "jump" to the nearest full step. If one moves one way, and the other moves the other way, then they'll be twisting the gantry out of aquare, although the amount would be very, very small.

If it's as rigid as you say, then one switch would be fine. But you need to make sure that the gantry is perfectly perpendicular to the axis is moving along.
If you go with two switches, you need to position them so that they get the gantry perfectly square. Both methods require very precise measuring and fabricating.