OK yes I can see how that would work. For info, I have been emailing Balazs at CNCdrive today and he has been replying pretty quickly. Impressive given it is Sunday afternoon/evening, and a bank holiday (in the UK at least) ! Anyway I asked what would happen if 2 stepper drivers were connected to a single step/dir output E.g. if I went with the compact AXBB-E route which can drive 4 stepper drivers as standard, and connected both of my 2 Z step/dir to the same output to drive a total of 5 steppers. This saves the additional BoB, ribbon cable etc. In this scenario I would have:

1: X axis
2: Y axis
3: Z axis & 2nd Z axis from same step/dir outputs
4: A axis (slave for X axis)

The answers from him were when using one output to drive 2 stepper drivers:

Homing could not be controlled independently - well that is OK for me as they are either side of the Z axis which can't rack

Issues for rack and pinion as often one side needs to rotate in the opposite direction - again that is OK as both turn the same way for me (plus I could swap 2 of the 4 motor cables)

In theory one output on the AXBB-E could drive 2 step inputs on 2 stepper drives since the output can drive 20mA current max. and usually one stepper drive requires only 5-10mA of current for the step optocoupler, so 20mA should be OK. - this was my main concern but sounds like this will probably work


So in summary I'm prepared to give the AXBB-E a go, double up on one step/dir output to drive 2 Z steppers, and if I do run into problems then I'll have to add the extra BoB for the 5th driver.