Not possible, different networks, not compatible. It is possible (assuming it is the same network, both are CDMA for example) to swap carriers if the phone is unlocked, but not at the same time.
For work, I use Nextel (iDEN) and for personal I use Alltel (CDMA). Is there a phone that will handle both carriers?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic