Prusa Z axis stalls when connecting 2nd motor?

Hi guys.
Trying to revive my Prusa from the dead. Been there for 3 years now.
Problem is: the Z motors seam to stall. So.. I desassebled them, got 2 new motors and connected them direcctly to Ramps. Just the free motors not connected to the mechanical printer.
Problem in testing is the same with the old and new motors: works fine on one motor, but when I connect the second motor it stalls?
both motors are connected the same way, and they both coonect to the Z connetor on the ramps with a 2 in one connector..
I've changed the motors, the connetor, I even changed the polulu driver on the Z axis.. What else can it be?

You will either need to increase the current on the polulu with the trimpot (carefull steps and make sure you have a heatsink!)
or you can reduce the maximum Z speed in the firmware

thanks, i remembered the trim pot on the polulu. it worked alright.