There are plenty of reasons why someone would want to have a two SIM cards running in their iPhone 4 or 4S and most of them are legal! However going about this is not as difficult as you may think especially with the help of Vooma!
Check out the Peel PG92 iPhone 4 or 4S case from Vooma, it may look slightly more chunky than a normal case for the iPhone, but there is a good reason for this. Built into the case there is a battery and a very basic mobile phone.
Once up and running in conjunction with the Vooma App, this additional phone line works almost seamlessly with the iPhone, it uses the same dialler etc, so what you are getting here is a backup battery, additional line and the iPhone is jailbroken all in one go.
Source [Oh Gizmo]