How to repair mysql tables using myisam storage engine from multiple repairing threads?
Read carefully:

MySQL 8.0 Reference Manual  /  ...  /  Server System Variables


  If this value is greater than 1, MyISAM table indexes are created in parallel (each index in its own thread) during the Repair by sorting process.


If your table have 2 indices - then 2 threads will (must) be used.

PS. I'm not sure that you will see this in processlist... this has to be checked.