Well, you could get yourself something like this:Trusted Reviews: Draytek Vigor2820n Wireless ADSL2/2+ Firewall Router
but note that the second WAN socket requires that you have an ADSL modem actually maintaining the connection.
It does also allow a USB data stick to be connected to supplement the connection too.
I think this is the nearest you'll get to what (I think) you're trying to do. If you insist on two modems built into one box, I think you'll be looking for a long time, as that's not something I've ever seen.