I've split my resources between Amicable and PrimeGrid, allocating 20 to Amicable, 100 to PrimeGrid. And still, last day for example, I've had

Amicable 300.000
PrimeGrid 35.000

That doesn't seem fair... Am I missing something?

Thank you.

1) Credit points from different projects can't be compared directly.
2) Are you sure BOINC actually splitted run time between two projects the way you intended?

1) OK, I do realize that Credit points from different projects can't be compared directly, but still...
2) Apparently the split did NOT respect my intention, and Amicable Numbers continued to run as top consumer most of the time. I've ended up with not accepting new tasks from Amicable Numbers for now, and will 'release it' only from time to time, for short periods.
Might be a boincmgr issue, although I suppose that the project and the manager need to have some 'negotiation' on this, so both parts 'do their thing right'.
Without any idea about the source codes, I speculate that maybe the fact that Amicable Numbers uses all available CPUs at once makes it so different in behavior to the PrimeGrid [sub]projects and could play a role in boincmgr failing to allocate resources as displayed.
Thank you.

You can also limit number of CPUs used by Amicable Numbers here: https://sech.me/boinc/Amicable/prefs.php?subset=project - set "Max # CPUs" to whatever value suits you. BOINC should automatically schedule all other CPUs to PrimeGrid tasks (in your case). It should work better than split settings in BOINC client.

BOINC Manager splits by processing time. It doesn't know how much credit a certain task will receive. Also the resource share number is a long term thing. It won't average out the crunch time right away.