Actually it does move in both directions at the same time when you are controlling it with a joystick, although I suppose that could be changed.
I don't think I see anything wrong with the circuit as far as wiring goes. One of the earlier circuits actually used relays instead of the stepper power control board, but I was worried about the electrical / mechanical life span of the relays. They would potentially be turning on and off over 1000 times per day, which means that they could wear out in a relatively short amount of time. Of course, I may have been overly concerned.
The Stepper Power Control Boards are probably going to become obsolete in the near future, which may make this whole conversation redundent. I've recently put together a driver board design based on the Big Easy Driver and customized it specifically for this project. I have assembled the first prototype and it seems to be working. The thing that I realized along the way though is that I can actually provide the entire driver board for each machine for only a few dollars more than the Stepper Power Control Boards! If you only need to control a handful of machines, it might even be a bit cheaper over all.
It should also be possible to use both the Stepper Power Control Boards and the new driver boards in the same circuit too, so if you have Stepper Power Control Boards already then you should still be able to use them with the new system.
I need to put together a proper post about them, so I watch out for it.