I have a system with a Asus DSBV-DX motherboard. It was equipped with one Xeon 5420 (4-core) CPU, and it ran very stable on one CPU. After adding a second Xeon 5420 CPU the system was going into a continues reboot, pausing in the GRUB menu.
After some research I am able to boot the system by adding the "nolapic" option during boot time. The problem is that the system then boots with only one cpu, and of that cpu, only one core active. I am deepdiving on all available material on the web, but so far I have not been able to get the system running with a dual Xeon setup.