[gnea/grbl-Mega Issue#89] Arduino Mega GRBL – Stepper motor run on it’s own

Hey
I have a strange problem. I’m in the middle of building my own CNC, and I have connected the Arduino mega 2560 to the stepper drives as shown here: https://imgur.com/a/rI6zJyd

If I just connect one stepper motor to this setup I can control it as I should be able to.
There is a bit of chatter from it, and it doesn’t move smoothly, but I think that comes down to finetuning of the stepper perimeters.
When I connect a second stepper motor the first one just start spinning fast.
I have tried with different stepper motors and every combination of the stepper drivers.
What could be causing this?
I’m at a lost right now…
Thank you in advance!