Hello Marianne,

This is not an issue I've come across before but the following TechNote may help:
http://seer.entsupport.symantec.com/docs/273511.htm

The key to this configuration is to understand that each storage unit needs to have the Maximum concurrent drives used for backups parameter configured for fewer tape drives then what is configured for the media server in Media and Device Management. If this step is missed and the setting is configured for all the tape drives in the library, the first media server in the storage unit group will perform the majority of the backups.

Thanks Dustin

Pure co-incidence - the configuration is identical to the example described in the TechNote:
6 drives shared between the master and 2 media servers (all drives confgured on all sytems in device management). 2 media server stu's configured with max 2 drives each and grouped into stug (with load balance configured). Mpx values same on both stu's. Policies that generate most jobs are going to stug. Unfortunately a 'balanced load' is not seen in practise...