No, it is not possible. If you noticed, the sim card tray is too small to hold 2 sim cards.
Originally Posted by Marlonuc
Hey i had the same idea and found the product for this.
I am using Hypercard for iPhone 3G/3GS from a company called 2-phones-in-1
The card works fine. I can select the sim under sim applications and the sim change is really fast.
And the best is i stll have full 3G signal.
Yeah... These things are out there... You have to cut up your SIM cards to fit them into one, but allegedly it works.