On a previous engagement we implemented this type of approach in a 100Mb
switched Ethernet environment to keep traffic on the backplane of switches
and to prevent interswitch traffic.  On the server side, multiple NICs were
defined at the UNIX level.  On the client side, the TCPSERVERADDRESS was
used to direct backup traffic to these different NICs.  Also, we were
looking into the use of multiple NICs from a single server with BACKINT/ADSM
to the ADSM server by setting up multiple client option stanzas and using
the ability of BACKINT to send some sessions out one interface and the rest
out another interface to improve parallelism and throughput.  In the latter
case, both the TCPSERVERADDRESS and TCPCLIENTADDRESS parameters would be
used.

In a normal client configuration, you would run multiple client processes
defining multiple nodes per physical platform, segregating filesystems
between the client nodes (via DOMAIN statements).