Ultimaker 1.5.7 PCB driving multiple stepper motors

[MattL 0]

This is my first request for help from any forum, so please take it easy on me. 

I'm building a monster (5' X 5' X 5') printer after owning two kits.  Because of the weight of the gantry system, I'm engineering it to have high torque stepper motors on each corner so my Z-axis will have 4 steppers.  

To compensate for the current draw, I've purchased adapters to allow for multiple motor drivers off of the single Z port of my Ultimaker 1.5.7 PCB.  My thought is to parallel only the data signal off of the PCB and use a separate power supply that can handle the load to power the drivers.  If this will not work, will a current amplifier do the trick?

Attached are pictures of the drivers I have.

If this has already been addressed, please add the link to your reply.

If there is a more appropriate venue for what I need, please let me know.

Thanks