Ok, maybe spell out what you have a little better.
You have two quad Xeon machines. Which Xeons? While I'm guessing some of the newer Core-based Xeons, you could have P4 Xeons or even P3 Xeons for all I know. The logistics of each situation is a little different.
From the work I've done with Hyper-V, ESX, and GSX (before it became Server) and more recently Server 1.0 and 2.0, with primarily Linux VMs, you'll see Linux VMs easily scale from 1 to 2 cores, but beyond a few specialized scenarios, it doesn't scale well to >2 cores. The Windows Server VMs I've worked with scaled even worse, though.
What you really need to ask here is 'WHY'. If you're just looking to see the CPUs pegged out, look to your workloads. Find the bottlenecks *inside* the VMs. On the other hand, if you'd just be happier seeing the CPU usage pegged out, just install F@H and forget it. If you're looking for maximum performance, you're probably as close as you'll get when you have the 2 VMs and 70% utilization.
CPU utilization isn't everything. Look at your ram and disk load levels, too. If they peg out early, no number of additional VMs will give more work.
Behemoth - media server
Galactica - test lab server
Falling Outside the Normal Moral Constraints - MBP 15
Donnager - M4800 DTR
Rocinante - E5530 testing machine
Razorback - E6430 ultraportable