|Have you tried using it as "just a router" yet?|
If not, plug the modem in, and then plug that into the "external" or "WAN" port on the 2wire and see if it doesn't work.
I have a 2wire combo unit at home too but I've never tried what you're asking about so can't say for sure if it will work or not. The worst that happens is it doesn't and you have to spend another $50 on a SOHO Router.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.