with GRBL you really only have X, Y, Z.. so you just pair up any signals you want you can chain together multiple drivers on any axis no problem.
Its not a issue as its only the control side, power the motors up with the lead screw couplings undone, so the motors can sync with no load - position your axis's by hand and do up the couplings I run 2 Y axis drivers and motors and have just set them up My double wide CBeam
