I don't know anything about any balancing involved, but the process that
Windows uses to bind the two channels of an ISDN line is extensible to bind
other connections too.  I've seen a system uploading a file to a web host
using two ISDN channels, a 28.8 modem channel, and a 33.6 channel; using
three different ISP's, and data flowing on all four lines.

Surely if Windows can do that, we can come up with something for at least
outbound use of a DSL and a Cable line.