Win98SE had this functionality built in, I'd assume 2k, ME, and XP do as well (I've not gone back to a modem since 98SE, so I don't know if they do or not). Pretty easy to setup even, just plug the modems in and tell them to bind in the appropriate window.
I think the second line dials on demand as well, I could be wrong though. It's actually a good solution for an office who can't get broadband but needs basic internet access. Setup ICS on a 2k box with modem binding and dial on demand.
IIRC, a guy I used to work with tried to bind two 115k ISDN connections together, because he needed the speed and had no access to broadband. I don't recall if he ever got it working or not.