Unitrends Hyper-v 2008 R2 backup

I recently purchased a Unitrends Recovery-833 appliance and so far have had great results.  One thing that I did not try during my trial was to backup multiple VMs at one time and I'm experiencing something that Unitrends support is telling me that is a Microsoft Hyper-v limitation.  If I select all of my VMs that are in the cluster, Unitrends only backs up one at a time and queues the rest.  Has any one experienced this?  If so, do you where the limitation is within Hyper-v?  Also, is the limitation in Hyper-v 2012 R@ also?  Thanks