
All change please!
> It could be a simple SIM issued from one of the two networks with a roaming agreement, although the most likely mechanism for this is a dual-IMSI SIM. This would give access to both networks, but it would not switch on the basis of the strongest signal.
But it could switch on the basis of a command from a Google server - and since Google have very good records of signal strength by carrier for every location that an Android phone user has ever visited, I suspect that this won't be a problem.