I did that for about a month when I used Win98SE. You have to have 2 modems, 2 phone lines, 2 Dial-Up subscriptions, as well as make certain that the ISP providing both accounts will support a shotgun connection (You may have to describe what "shotgunning" is to them, sometimes those guys can be quite moronic).
Though that's quite unessecary. 2 x $14.99/month (if you're lucky) is $30. You could pay $20 more a month, and get 7 times the connection of 2 x 56K (768 DSL). =P