I can shed some light on that one. I have a MSI dual-P3 733 and if one CPU runs SETI, the work-unit times are in the 8-hour area with Crucial CAS2 PC133 at the fastest BIOS settings. With both CPUs running SETI at the same time, each CPU gets them done in about 10 hours or so. The reason is that SETI relies heavily on the memory and north bridge, and when both CPUs are sharing the memory bandwidth they each go a little slower.
By the way, if you're not on a team, please join up with the AnandTech SETI team.

We're racing DSL Reports in a 100,000-workunit drag race right now and we're about 8000 WU's behind! Check
this thread for the current report and have a look at the real-time
race chart.