Actually, there are dual SIM phones... e.g.:
http://www.unwiredview.com/2007/10/02/samsung-d880-duos-dual-sim-card-phone/http://www.mobile-t-mobile.com/mobule-reviews/dual-sim-phone-philips-699.htmlSome can have two SIMs, but are using only one at a time and allow you to choose which SIM you want to use when you do a reset or a switch between the SIMs.
However, in case both SIMs are active, the phone actually has two radio sub-units and it is essentially two phones with one UI controlling both the "phones".
HTH
Abhik.