Sean,
we're also testing with clustering. I tried to solve the problem this way
(I'm sure it's not an optimum but it seems to work):

On both machines the ADSM client is installed with the scheduler service
running.
Server1 or server2 have a local c-drive and can access drives r, s and t
alternatively.

On server1 I have a dsm.opt files with
domain c: \\cluster\r$ \\cluster\s$ \\cluster\t$
On server2 only 
domain c:

So both c drives are backed up and all cluster drives, too.
When server1 has a drive, backup is a bit faster than when server2 has it.
In testing I didn't find great differences when backing up over a TR16
network.
Be sure to use a domain user for the scheduler service account. Otherwise
you don't get the "network drives".

Good luck,
regards, Michael