There are quite a few areas with two TDMA and two AMPS networks. Depending on the phone, it can be easy or difficult to switch it between networks. Some phones require being hooked up to a computer to be switched, and some can be programmed over the air.
In areas where both Cingular and AT&T have TDMA networks, the networks are still separate (labeled orange for Cingular, blue for AT&T, but it is still A & B for AMPS).
I've recently activated several old TDMA phones for use on Beyond Wirelessor CallPlus. These have to be phones that were either previously on the AT&T TDMA network, or that can be reprogrammed over-the-air. I.e., they can't activate a locked Cingular TDMA phone that can't be reprogrammed over the air.