Active dual sim is having 2 sim cards which both sim cards are active at the same time, which means you can make and receive calls simultaneously. Rebel 2Phone for iPhone 4 finally can use two sim cards on their iPhone 4 simultaneously. Thanks to hardwo

Active dual sim is having 2 sim cards which both sim cards are active at the same time, which means you can make and receive calls simultaneously.: "Rebel 2Phone for iPhone 4 finally can use two sim cards on their iPhone 4 simultaneously."

Seems like a good idea if you really need to have two active SIM cards. But according to this link, the phone needs to be jailbroken.