Good point ... I didn't set this up, but it looks like the STUG that is used for backups is set to use the "Media Server Load Balancing" option and the STUG that is used for the duplication uses the "Round Robin" option (basically the SLP uses these two STUGs to do the backup/dupe).
I suppose this would only work (work is a relative term I guess, the duplication is working, but going over the ethernet network vs. going over the SAN) if the backup happened to be on the same media server that was used in the round robin. Or heck, maybe this can't work at all. :)
Ah well. Back to the drawing board. Maybe a support call too.