Each D8B Console (the mixer unit part, not the rack unit part) has a chip in it, that has a serial number permanently written into it.
Mackie OS 5.1 will check for that serial number every time you boot up the computer, to make sure it matches the serial number that was used to activate the Mackie OS 5.1 itself, and also all the plug-ins.
I guess if you were to authorize two rack units (or "CPUs" as you call that... technically, the CPU is just the main processor inside the computer, though, not the entire computer... always confusing to see a whole computer unit be called "CPU") using the same original "Console" serial code for activation, you could probably swap out rack units without getting an activation prompt, as long as the Console stays the same. (Once you'd try to use those rack units with a different Console, they'd all give an activation prompt again, though).
Mackie OS 3 doesn't require an authorization for the OS itself - but the plug-ins will still compare the serial number and give you an activation prompt, if the console changes, or a rack unit for a different console is connected.
IMO, the best way to have more than one D8B rack units available, that work with any console, regardless of their built-in serial number, is to use the hacked version of the Mackie OS 5.1. This circumvents the serial code check, and as a result, you can use a D8B rack unit with the hacked OS on it, with any D8B console you want (or have more than one rack unit ready to go, so you have a spare... that's how I have it, here).
The hack that is easiest to use, can be found here:
viewtopic.php?f=2&t=1634&p=15134&hilit=MackieOS.exe#p15134In the ZIP file that is attached to the post, there's a file named "MackieOS.exe". On a fresh installation of the Mackie OS 5.1 (don't try with an older Mackie OS version - this will most likely fail), replace the file "MackieOS.exe" on your D8B harddrive, with the file from the hack. Then restart. After that, there will be no OS authorization prompt, and all the plug-ins will show as authorized.
(I had nothing to do with the hack, but this type of question comes up very often in different shapes or forms here on the forum... the hack above is often the answer).
Best of luck!