Hi, as I wrote some posts above Cycles GPU use ~ one CPU core for each GPU.
I have two GPU now and it works as expected, tested with Blender 2.91.
Can you please add your OS and Blender version.
Blender 2.92 provide GPU+CPU for Optix cards, too.
Iirc your GTX 1660Ti can use Optix features and may you have to disable CPU there.
Blender should not crash anyway, if you can reproduce it it is may time to file a bug report.
Windows 10 20H2
Intel i5 3570K 16 GB RAM
GTX 760 4 GB /Display card
RTX 2060
Driver 455.32.00