IPad – Using a different SIM card on iPad2 3g while travelling outside the US

No - the sim card only provides an ICCID number to the phone. That number is read once per boot as the phone just uses the sim to identify what account to bill for service when registering with the nearby cell towers (IMEI and ICCID together let the system know what number to assign your phone and who to bill).

Something else must be causing your observed behavior.

Keep in mind, there is no guarantee two phones in the same location are not on distinct cell towers. Especially if one tower is overloaded or doesn't have high speed data so the phone /carrier might pick a faster connection that happens to have less bars of signal.

What does your carrier say if you call them for a site survey / diagnostic?

Afaik Verizon can only unlock the phone completely and not block certain carriers.

Until the iPhone 5, there was no model that supports CDMA and GSM.

The reason your phone doesn't work on an AT&T network (or most international networks for that matter) is because AT&T doesn't run a CDMA network.

http://support.apple.com/kb/ht3939