Though there are dual-SIM card adapters available for iPhone 4, but they sure aren’t as classy as this dual-SIM case. We’re also hearing that iPhone 5 may actually offer dual-SIM by default. But those of you who can’t wait till the release of iPhone 5 for the dual-SIM functionality, can probably try this new iPhone 4 case called "Rebel 2Phone Dual SIM Standby Case" by Rebel SIMCard, which not only provides you with an extra SIM slot, but also offers you a secondary battery of 800mAH.
Once you have the case, you may have to install "Rebel 2Phone" app on your jailbroken iPhone. The app will let you make and receive calls with both the SIMs. It will also add another battery and signal strength indicator for the secondary SIM.
If that sounds too complicated to read, sit back and watch this being demonstrated in the video below.