Geeetech

Mine was advertised as a Green tech but who knows with these Chinese models. Anyway I've built the printer but my issue is with the z motors. I have two motors but only one control on the controller (GT2570 rev A) with only one spare and the for a second extruder. I've tried connecting them in parallel but the both just stall and grind. Each Z motor works perfectly independently. So my question is why they use a board that will only control 3 motors when the printer has 4 motors and how to fix it. I guess wiring the on series could work but wiring diagrams on the net show different colour wires and would mean cutting wires etc. Not sure if I'm missing something completely but any help would be appreciated. Both z axis move quite freely and set a correct height. Thanks