I tried this idea with Cable, and Modem.
I started the Cable in one Window of IE5.5, and opened a second Window of IE.
I started the Modem; the modem connection takes over both windows, and neutralized the Cable.
Though, I have two I.E running the Registry setting for IE is only one, and IE prefers the Modem.
Could be that if I installed Netscape, and assign it to work with the Modem, and IE to work with the cable it will work (but I am too lazy to try).
So if you install two NICs assigne one to The Cable, and one to the DSL.
Leave the DSL with the original Software, (mine came with Netscape), and PPPOE dialup.
Who knows??? May be it will work.