First, are you confusing Cingular with Nextel? Nextel is a part of Sprint, Cingular is a part of AT&T. Your password for AT&T won't work on Sprint at all.

Second, the different technologies would then kill you. AT&T uses GSM, Sprint CDMA. You'll never see the Sprint network on a Cingular card or vice versa, unless it searches for both types of networks (which is unlikely).