What? Oh, wait, got it.
I'm Sorry, I don't understand, maybe it's the fact that there is only one punctuation, or maybe It's all the "and and" stuff. Could you re-write it?
I got you and your friend buying the new iPhone 3GS and, wait I think I got it. Okay, adding two more lines to your plan (iPhone 3GS's) and then canceling one of them, moving your original SIM card to the one you canceled?
If that's what you meant, then... I don't know. It would work fine, but I thought if you "return" the service, you have to give the phone back too. I'm not sure.
Basically, If you can get ahold of a new, not-on-a-plan, iPhone, you should be able to put a sim card in it, and have it work.