I don't know about CDMA vs GSM, though I think it might work (there definitely are two-GSM-SIM phones, and GSM-and-CDMA doesn't seem philosophically different).

I don't know about CDMA vs GSM, though I think it might work (there definitely are two-GSM-SIM phones, and GSM-and-CDMA doesn't seem philosophically different). Having a phone work on both AT&T and T-Mo, though, is easy, Galaxy Nexus or Nexus 4 can do that, just to name a couple of popular ones.