I've not read this thread as clearly as I should (so this may have already been pointed out), but if network A has the Internet connection and network B is the remotely-attached network, if you expect to be able to connect from computers on net B to computers on net A (or vice versa), you'll have to set up a static route somewhere, and that's probably going to be a bigger pain than it initially seems.

Of course, if you're not interested in direct connections between the two networks and just want to piggyback their high-speed connection, then that's not an issue.