Hi
I have seen a similar situation and where we had a loopback interface configured with one /32 public ip and done the overloading over that.
with respect to routing is concerned u can have 2 defaults with and without admin distance (floating static route) to have the auto internet traffic fall back mechnaism to the second T1 when the first T1 goes down.
But u need to make sure that ur loopback is being routed properly on the both the links on the SP side too withtout that u will face problems in the return traffic to ur network...
regds