Android DOES support dual-SIM phones, so don't place the blame there. Just Google it, you'll find Android-based dual-SIM phones. Just not sold by AT&T, or TMobile.

I'm not sure why AT&T doesn't carry any, and maybe they soon will, now that they're using Enterproid. There's no reason to say that your two SIM cards won't both be locked to AT&T. You pay for two plans, but only carry one phone. Seems like a win for them.

It may have been the intent to have separate lines for Clear customers, but I know that wasn't the case at the Albany, NY airport. There, Clear customers just got the skip to the front of the line, just like airline / airport workers. They still went through the same check lines as everybody else; just didn't need to wait to show their ID.