The good news is that you can use 2 Sim Cards on your iPhone, simply by switching on the second — External Sim Card. You can easily do that using this dual sim adapter for iPhone 4 / iPhone 4S.
How Dual SIM Adapter Works on iPhone 4 / 4S:
A Sim Card for connection enters inside the default slot, and expands away from body of the phone. This specific external piece is effective at holding two sims. Don't forget that just one sim card can work at the time.Dual SIM Adapter for your IPhone can be purchased on Italian web store — Vaveliero.com for price of 49.90 Euros ($ 64). Finally you found solution for carrying two phones at a time?