Thanks to a special accessory, sold for thirty dollars, you can use two SIM cards simultaneously on the iPhone 4. A system that has existed for many years and now becomes compatible with the new phone's SIM Micro Apple.
A special cover allows you to connect to a normal two sim adapter that will fit in the space micro sim. 
Although this gadget is interesting, it does not allow simultaneous use of the sim (which is common for these adapters). You will then click through the menu, which sim to work on your iPhone 4. 
If you are interested in the product can be found on this site . You might also like:
Dual Sim for iPhone 4!
iPhone 4 Unboxing And Disassembly Video
GreenPois0n to Jailbreak iPhone 4, iOS 4.1?
First encounter iPHONE 4G SIM modification and ACTIVATE
MuscleNerd: One Step Closer To The iPhone 4 Unlock