i think it would work. If i am correct, the DSL comes down a separate part of the phone line and the modem would be using the voice part of the phone line.
the part that might get confusing is telling your apps which connection to use. For example, IE (as far as i know) can only be setup to use one connection type. You cannot have one IE window open using the dial up and another open using the DSL.
Let us know how it goes.