That's what's called an Engineering Sample. Not supposed to be sold and are supposed to be turned back in when testing is done. I won't speculate on how you or "the guy" got it. But it's pretty cool to see it with the trackpad.
I know 2 people that are testing out Pro's right now and can't figure out why all the cases/covers have a cutout in the middle of the back... it's for the trackpad.
Back to your original question... I don't think you can use it on both carriers at the same time. You'd have to switch back and forth. At least that was my experience with someone with an older phone that could switch.