I doubt that was the problem. I also have a premier account and had no issues ordering two via the app.
Likely just server overload.
I think AT&T has something screwed up in my account. I tried putting my wife's phone into the iOS app's upgrade check form and it worked fine. Both phones are out of contract and part of a shared minute/data plan. Weird.
One annoying thing: I can upgrade her line with 1-3 business day shipping. Too bad she doesn't want to. Guess I'll bid my time waiting for the one coming from AT&T. :/