If your CPU has more than 1 core, it should be doing it automatically, unless you have RAM restrictions. Choose the Maximum Output profile and BOINC will use whatever it can.

Because some long tasks from Seti are forcing WCG tasks to wait for days to run. Suppose it would only be a problem if tasks had to wait so long they went beyond the report deadline - not sure that this would ever happen.

Basically I asked out of interest...

BOINC switches between tasks periodically if you are attached to multiple projects. However, other things such as deadlines and checkpoints are taken into account, so it may not work how you expect. You can change the switching interval in your preferences.

This can happen, especially if reporting deadlines are wildly different between projects. As time goes by, the WCG would be closer to the due date, and get more and more computing time. The longer project would then wait. BOINC manages things well if you give it enough time to do its thing.

Mixing projects works best if the due dates are about the same. Since the BBC world climate projects can run for months, they tend not to mix well at all. On the other hand, some of the mathematical projects have due dates of a day or several hours, and the workunits can run seconds or minutes.

Once you have mastered how Boinc works you can start to get into some of the advanced settings that some of the other projects have that I have not seen here at WCG.

On the other projects in your account settings for that particular project you can set the "Resource Share" which can give/take how much time that project uses your processor.
Let's talk about a single core machine for example. You have four projects that you are contributing to. Assuming there is plenty of work available. By default that is 25% of your processor to each project in a 24 hour time frame if all of them have the same resource share level of 100 (default=100). If you change one of the projects to 50 and leave the rest at 100 then you will have one project using 12.5% of the processor and the rest using 29.2% processor.
If you have a multi-core processor or a ton of projects then it can get confusing. Especially once the projects run out of work and the other projects keep working. That is when you start getting into long term debt. Short term debt is when you have a project with no checkpointing and running for hours till it ends and all of the other projects could not run causing the short term debt. Once a project with Long term debt has work available Boinc will give that project priority over the others so that it can work the debt down. It will still switch to other projects after the checkpoint but it will switch back to the long term debt one more frequently till it balances out the debt to the resource allowance.

It's not perfect because us humans get involved and tell Boinc to suspend a project or No new work or something else. Just click at the buttons. What's the worse that can happen? Reload of software, rerun of the wu? So. If you click the buttons and learn something new and how it works, great. If you just sit there and watch the counters or graphics, your not learning.

Good Luck, I would tell you to go to another website and learn more but I am sure it has been mentioned or you already know of those sites.