They're meant to be mounted so that the display on the light is facing the front of the tank (e.g. parallel to the sides). It's really just for aesthetics and convenience of being able to read the display on the light.
You can mount them in either direction -- parallel to the sides or parallel to the front -- whichever gives you the best coverage for your particular tank.
If your using the wire hanging kit, mounting either way is easy. (It's what I used.)
I haven't see/used the mounting bar though, so I don't know if it gives you the ability to mount in either direction, or just one...
That said, with your tank dimensions, if you plan on doing SPS across most of the tank, you'll probably want a 3rd one with all three mounted parallel to the sides (e.g. displays facing front).
If you plan on mostly LPS or just a bit of SPS beneath the lights, the two mound parallel to the front may be enough.
