I remember the 2 lines idea from the days of diap-up. People used two modems and two phone lines not isdn lines either. I don't know if there is any special software, but it was the days of Win95 and Win98 so there is likely built in support in XP. Unless the support for two isdn connections is built into the modem and not the drivers you should be able to just select the two phone connections instead of the isdn connections you normally have like Repitah said.
Just a note though. Because of the way the protocol works you won't be able to download a file using two connections unless you use segmented downloading. A connection is first established to the server starting at a specific point in the file if resume is supported. The data is then requested sequencially and you can't skip a fragment or request a prior fragment without a new connection to the server. With web pages the elements will be downloaded equally on the available connections. With single segment downloads it will only download the file using one connection.