Y with A as slave axis won't work

[Neale]

If you wire two stepper drivers in parallel like that, there is always a risk that you will have problems. It is a theoretical risk (and I am not going to describe why it might happen) and in practice it often works. That does not mean that it is a good idea!

In particular, because the IP/M has a separate output that you can use for the slave axis, and it is very easy to configure in the software, it is just as easy to wire the two outputs to your drivers. I would strongly recommend that you do it correctly.

Have you thought about how you will square the gantry when you home it?