I have two machines which have 24 cores. If I assign them to do Milkyway, they get the largest multicore tasks possible from that project - 16 core tasks. This leaves 8 cores free. Couldn't Boinc either:

Ask for two 12 core tasks.

Download from my second highest weighted project some single core tasks?

Use a app_config and set the MT task to use 12 threads per task so that you can run two tasks per machine.

Or set to use 4 threads per tasks and run six tasks per machine concurrently. Just have to set the MT application up correctly.

The BOINC documentation is here. https://boinc.berkeley.edu/wiki/Client_configuration#Application_configuration

The section on app_config gives an example.

<app_config>
   [<app>
      <name>Application_Name</name>
      <max_concurrent>1</max_concurrent>
      [<report_results_immediately/>]
      [<fraction_done_exact/>]
      <gpu_versions>
          <gpu_usage>.5</gpu_usage>
          <cpu_usage>.4</cpu_usage>
      </gpu_versions>
    </app>]
   ...
   [<app_version>
       <app_name>Application_Name</app_name>
       [<plan_class>mt</plan_class>]
       [<avg_ncpus>x</avg_ncpus>]
       [<ngpus>x</ngpus>]
       [<cmdline>--nthreads 7</cmdline>]
   </app_version>]
   ...
   [<project_max_concurrent>N</project_max_concurrent>]
   [<report_results_immediately/>]
</app_config>

The key part is just specifying the --nthreads value you desire.