If you don't want to bother with WF, scheme you need could be done by
- create a new SP specifically for Exch DB and MB backup
- limiting the maximum stream in SP to only 1 stream
- Create an exch DB subclient, and an exch MB subclient, each with 1 stream
- create schedule for DB subclient to start ealier than MB subclient for 1 min.
Once you follow above steps, you see that DB will get backed up first, and MB backup wait until DB backup finish.