Normally when you move 2 VM’s from one host to another using Live Migration they move one at a time. Yes, the VMM job pauses at 50% for the second machine for a while – that’s because it hasn’t started to replicate memory yet. The live migrations are serial, not concurrent. The memory of a running VM is being copied across a network so the network becomes a bottleneck.
I ran a little test across 3 Windows Server 2008 R2 Hyper-V cluster nodes to see what would happen. I started moving a VM from Host A to Host C. I also started moving a VM from Host B to host C. The first one ran straight through. The second one paused at 50% until the first one was moved – just like moving 2 VM’s from one host to another.