Greetings, I'd like to allocate two of my CPUs to one project and two other CPUs to a different project. I've spent a lot of time adjusting my computing preferences and it seems that I can only get one project to run at one time. Please let me know if I'm missing something - thanks in advance.

The BOINC scheduler will average them out over time, so that you get the percentages that you set via the Resource share.
But it often does not run two projects at once, but rather runs them at separate times.

If you want to run them all at once, there are a couple of ways. The first is an "app_config.xml" file in each project folder, specifying the number of work units of that project to run at a time. It looks like this, and you can choose the number of tasks that you want.

<app_config>
   <project_max_concurrent>1</project_max_concurrent>
</app_config>

You create it in a text editor (Notepad), save it as an ".xml" file (not a .txt file), and place it in the project data folder.
For example for MilkyWay: "C:\ProgramData\BOINC\projects\milkyway.cs.rpi.edu_milkyway".
Then, you activate it by restarting BOINC, or rebooting the PC.
For more info:https://boinc.berkeley.edu/wiki/Client_configuration#Options

Another way, somewhat more complicated, is two create two (or more) BOINC instances, one for each project:
https://www.overclock.net/forum/18056-boinc-guides-tutorials/1628924-guide-setting-up-multiple-boinc-instances.html

However, each of these ways has various pitfalls, and are best used in special cases when you need them. Otherwise, just use the Resource share, and let the BOINC scheduler handle it.

Okay, makes sense. Thanks for your help.

There is a way to make the adjustment for a new project converge faster to the correct value.
That is with the use of a "cc_config.xml" file placed in the BOINC Data directory (C:\ProgramData\BOINC).

It looks like this:

<cc_config>
  <options>	
  	<rec_half_life_days>1.000000</rec_half_life_days>
  </options>
</cc_config>

Someone said recently that it is not necessary for the newer BOINC versions, but I don't have any info on that.
I always use it when setting up a new machine.