You can run 2 network cards for 2 different internet connections, but you can't have a download run off of both connections at the same time to make it go faster.
You could have 2 files download (1 on each connection) in order to max out both if you wanted. The easiest way i can think of to do that in windows would be to use forcebindip (
http://www.r1ch.net/stuff/forcebindip/ ) and launch a browser to the NIC that windows considers 'secondary'. Then just open up a second browser as normal to download off the primary connection.
Unless whatever program you're using to download has the ability to choose which adapter it uses internally.