Using one signal to control 2 drives is not recommended. It as nothing to do with the current setting on the drive it's to do with Resonance and how the drives compensate for resonance.
If using just one signal then resonance compensation on one drive can throw the other into resonance and vise versa. This can lead to poor performance, stalled motors or lost steps or mixture of all 3.!!

It's standard practise to use the Limit switches to square the gantry.
The way Mach and most Motion control cards home is to decouple the slaving feature while homing and drive each motor individual but simultaneously towards the home switches. Which ever switch is triggered first that motor is stopped and reverses off the switch while the other keeps going and does the same. When both motors find a switch the homing is finished.
In practise this is very difficult to see. If Gantry is square and switches are correctly set then Both motors will stop/reverse exactly together in one seemless action.

Also the Relay will be fine with 2.2KW spindle. I think you maybe don't fully understand what the relay is used for.?

It's only used to switch on/off low current signals that control the VFD. You don't use it to turn on/off power to the VFD.
Cutting power to the VFD while spindle is spinning is not recommended so never do this. Always stop the Spindle with the VFD Stop input then remove power.
Also NEVER put any switch or Relay between the VFD and Spindle other wise you risk damaging both.

Paulro's problem was a simple set up issue and slight wiring problem go check is thread to see why.