An end run around ATT?

I presently have an IP with the unlimited data plan that I will transfer to my new IP 4 when it arrives next week. I would like to be in the "tethering" business occasionally, but don't want to buy in to the 2GB deal I would have take and don't want to give up my unlimited deal. Now, if ATT would kick up the 2GB deal to say 5GB for the same price, I would probably be all over it.

So, I'm wondering if I keep my old 3GS with it's sim card, could I use both of the phones with the same telephone number? NO, I wouldn't have them both on at the same time. I would jailbreak the 3GS for tethering on my unlimited plan with the IP 4 in the off position. Probably 95% of the time, I would only have the new IP on.

Would this plan work? Considering what a used IP is worth, there would be several months before I would break even on this plan.