VMware is definitely a good option. If you put that on a dual-processor box (or just a P4 hyperthreading box), the OS's native multithreading will keep everything humming along nicely. VMware ESX Server runs on the bare metal, while the GSX Server runs on a host OS like Linux. My only experience is running the "Workstation" product, but it does work like a charm.

If you really want two separate CPUs, maybe you can find a co-lo vendor that offers "blade" servers. These sorts of servers are offered by many vendors. A typical offering would be 16 vertically-mounted mini-motherboards in a 2U or 3U chassis. They wedge laptop hard drives and laptop CPUs in there to keep power consumption and heat generation down. If you don't have particularly high performance requirements, this might be perfect for you.