Hi B_S,
Simple...create another job that backs up the other server, and disconnect the 2nd tape drive from that server.
Run the jobs to back up your SBE server and the 2nd server at the same time.
Either way you will need another Library Expansion Option (LEO)...BEWS will ship with a free license for 1 drive, but the other will not work until you license the 2nd tape drive.
You will need the RAWS agent installed on the 2nd server, and any application agents installed on your media server for apps running on the 2nd server (ie. SQL, Exchange etc).
Laters!