BOINC FAQ: Running more than one project or Work Unit simultaneously

From Unofficial BOINC Wiki

If you have a multi-processor or hyper-threaded Pentium 4 system you can run as many Work Units as you have physical and/or virtual processors.

[edit] Explanation

If you only have one processor, it does not make sense to try to run more than one Work Unit at a time. All that happens is the cache memory is split between the two Work Units and both run at less than half the speed of running one Work Unit. BOINC was designed knowing this and wanting to get the most out of your systems did not allow this. Therefore if you attach to more than one project, the system runs one Work Unit for a chunk of time (default is one hour but this can be changed via your Preferences.

If you have a hyper threaded processor, you can run more than one Work Unit at a time. This will usually be beneficial in terms of doing more BOINC work in the long run. Each Work Unit will take longer to complete but the time will usually be less than that for running two Work Units sequentially. To run more than one Work Unit at the same time, you need to set the preference 'Use at most x processors' to a number greater than one. You may also need to change your BIOS settings to turn hyperthreading on.

If you have two or more physical processors, you can run more than one Work Unit at a time. This will clearly be beneficial for getting more BOINC processing done.

[edit] Best Combinations

To crunch as much work as possible, you may find you get more performance gain if you run two different projects. The different projects may be limited by different parts of the processor.