Controlling vault - number of duplication jobs created?

I've recently configured vaults to roll data off of my MSDPs.  I have 3 appliances each with a MSDP and 3 physical tape drives attached via fibrechannel.  My first vault that I ran only launched 4 jobs:

#1 - 14TB - media server 1

#2 - 4.5TB - media server 2

#3 - 2.4TB - media server 3

#4 - 2.0TB - media server 2

Each of these media servers has 3xLTO5 drives attached.  So my vault only made use of 4 of the 9 attached drives.  How do I get Vault to break up these jobs into somewhat smaller ones to stream to *all* my tape drives on each media server?  I'd get through things more quickly with more numerous, smaller jobs.

Is this type of control even possible?

Thanks in advance!

-Mike-