I bought a days worth of unlimited 2g data and talk/texting from TMobile. I've diverted all incoming calls to it. The dongle remains connected just fine. At 11am Eastern Standard, my battery is at 76% which is lower than normal (particularly given the fact that my voice calls are going out bluetooth), but they were all clear as bell.

I'm not trying to use data. It is EDGE, but on the slower side of EDGE. Definitely your data SIM should be on the device SIM.

Another problem I have found is that when I tried to use a dual-IMSI roaming SIM (eKit), I could not access the SIM unlock PIN on it. On ekit, you enter 1-XXXX when you are outside North America to unlock it on its Jersey identity and 2-XXXX to unlock it on its US MVNO identity.