I don't think that would work too well. Windows doesn't allow the usage of more than one internet source unless you have software specifically designed for it, at least, that's what I've learned through trial and error. If your DSL is cut, then it'll switch to your modem, but that's only if the modem currently has a connection to the internet. Make sense?
Btw, could you cahnge your icon, gosh, that's gross.