Virtualizing boot camp with two different engines can have, well, strange results. Best to uninstall one before doing the other.
Giving it 2 cores on a Core 2 Duo will definitely cause performance issues - you'll have better performance with a single core. You can disable debugging in the advanced settings for the virtual machine.
When you say "uninstall one", do you mean uninstall the VM software's guest tools/additions/drivers? Or uninstall the software from the host?
I'd also like to know more about the number of cores. Is there a writeup somewhere explaining why that is the case? (I'm also wondering if that applies to other VM technologies; VirtualBox doesn't warn me against using both cores.)
Uninstall the other software's virtualization from the boot camp guest. You can leave the other system there, just not hooked up to boot camp.
All virtualization engines have the same problem - if you don't reserve at least 1 core for the host, you'll cause contention and performance issues.