Are you paying for both modems? If not, then I suppose it's not exactly correct to be on here advising you on how to make the best of it!
If you are, then you need a "bridging" router. I use that term because, frankly, I've made it up and I can't remember the real one.
Ideally it'd be a Cisco bit of it, but other companies do make much cheaper kit that will do load balancing across two connections and offer redundancy.
I can't really find a whole lot out there on Google - but that's what you need: a load balancing router.
