How to align to object in assembly mode, so they would have exactly the same height ?

These two shafts are exactly the same object if it is help it is a two shaft shredder with one motor on the left and the other on the right. I have difficulties to get this configuration. I specified the rotation arm of the gearboxes with a red box, these two should be at the same height.

2 Answer

make mate relation between the Grey couplings the ones in the red box and the top plane (tangent mate ) and they should be at the same height
I hope this will help

select the outer surface of the first coupling and the top plane and make them tangent
and the same for the other opposite coupling