It is possible to connect two modems together, but this all depends on if
the Kernel supports the modem yet.  (I dont know.)  But if you want to
connect two modems together, it requires that the phone line be connected
with a voltage source.  Usually a 9 Volt battery will do.  Thats only if
your in a pinch.  

Along the same lines they make what is known as a "programmers cable" which
connects to the built in serial port on the dreamcast.  The Cable should be
significantly cheaper than the Broadband Adaptor.  Though making the
Dreamcast a terminal for a firewall would be quite difficult using this
setup.

Good luck,
-=- Joel kershner -=-