You can change which media server does the backup by using Storage Groups.

Say you have mediaserverA (msA) and mediaserverB (msB)

You have a policy set up to use a storage unit from msA  (SUmsA). - this is a storage unit you already have setup and are using.
You also have a storage unit being used for msB setup already. (SUmsB)

Now go to Storage Unit Groups.

create a NEW Storage Unit Group
Storage unit group name "Tape -msA-msB" (this is a descriptive name that will let you know 1) that it is a SG for tape, and 2) what order the media servers appear in the group)
Storage units in Group "SUmsA & SUmsB"
Now choose you Storage unit selection
Failover (Choose the first storage unit in the list that is not down or out of media)

Save this

Now change your policy to use Storage group Tape-msA-msB

Now if it goes to use SUmsA and all the tape drives are down, it will fail over and start using SUmsB.