I'm sure the short answer is probably "no," but the first question is whether you are rate limited by your network connection for whatever you're downloading? That is, are you running at or close to the max speed of the DSL to begin with? If not then using a second network connection is unlikely to help you.
There's no way exactly what you want will be built in at the system level -- if you think of basic services offering you downloading, like getting a file via FTP or HTTP, the service on the other end will have no idea which of your two IP addresses to send each packet of information to.
So if it works for anything, my guess is that it would only be relatively specialized downloading systems like BitTorrent....