Perryg wrote:michaln,
Can you provide a case where this actually works? I can't seem to find any, and believe me I have tried. So far I can't find any combination that this actually works.
Theory is great but real world is what I have to deal with.

It is possible, and in heavy usage in the wild. I've attached a screen shot of my process manager showing three active guests -- two VirtualBox and one Parallels. I do this every single day as do, say, a dozen of my coworkers. Sometimes I have up 5 guests, mixed between Parallels and Virtualbox. Until week before last it worked on my main computer at work flawlessly. It still works today on my computer at home, no problem.
I can't figure out what changed on the work computer, but I'm now worried that it's going to hit my coworkers and home computer and impact our workflow signicantly.