Before you start you need a dual SIM holder. There are a few out there. There are ones that switch phone numbers when you shut down the phone and ones that can get switched on the phone. These SIMs require no jailbreaking or hacking to use if you have two AT&T SIM’s. All you do is cut both AT&T SIMs with an knife to a template size then pop both sims into the SIM adaptor and anytime you want to switch lines you just shut your iPhone off and turn it on.
Wow, I had no idea such a thing existed.