I'm not aware that you can do that - surely you'll still have to go through the 8.0.4 listener for the 8.0.4 databases.
I'm pretty sure you can have both running on the machine at the same time (since the 8.0.4 will be in the default_home, and 8.1.6 in a new home). You would of course need to use different ports for each listener, and update client TNSNAMES.ORA to point to the correct port for each database.