Two backup tasks to one external HD

I have a client that wants their nightly backup task to be shorter.  They have  server 2012 with one server 2012 hyper v vm and  one win 7 vm.  They feel that the backup task slows down the users login time when they all show up early in the morning.  

I would like to know if it is possible to do a backup of just the one server vm each weeknight and a bare metal backup on the weekends. Both to the same external hd.  

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David Lapello