Two stepper motors on one axis - help needed

I have a small micro gantry system which has two steppers driving the X-axis. I had it working a couple of years ago but it's been gathering dust since. I just went to resurrect it with a new install (2.4.6, upgrading now to 2.4.7) and I've run in to a problem. When I issue a G0 X1 for testing it works fine, but when I do G0 X0 to go back to the original position one of the X-axis goes in the correct negative direction, while the other continues to go in the positive direction. Any suggestions on how to trouble shoot this would be welcome!

Although I've used it a couple of years ago I only used it briefly and hadn't dug deep in to any of the setup, so basically I'm a complete newbie - I just know that it's worked before so should work now.

Thanks Andy. The wiring is definitely not the best and was a very quick and messy job. Now that I've got it relocated I'll do a proper job and report back.