Well, again, you could get something to work by setting multiple default routes on the machine that's actually connected to both Internet connections. This will probably do something. Most modern IP stacks will switch back and forth between them, but there's no guarantee of that. If you used a download manager that supported multiple concurrent downloads, it might actually do something to increase your usable bandwidth, but that's about the only place you'd find that it would work better.

If I were in your shoes, I'd at least try it.