You need a dual WAN router, sometimes known as a multiplexer.
Here's one now!
http://www.hawkingtech.com/prodSpec.php?ProdID=219
Double check that that specific product will let you use both at the same time. I think it will.
Hawking Tech makes great stuff, at really low prices. I bought their 8 port 1-gig switch and 5 1-gig lan cards from them, and should be receiving two hi-gain antennas for a wireless setup today..
Buy.com, compusa.com and newegg.com carry their products.