Mining Eth and Monero

Yes, you can mine Monero with your CPU while your GPU is mining Eth, at the same time.

I used to do it...just know it is not profitable. You will make pennies a day if you are lucky.

It's limited by the CPU cache, if that's not large enough it'll not be able to use 100% CPU power.

It's not really anything you can change, it's built into the hardware of your CPU. It's either large enough for the RandomX algorithm to be able to max out the CPU power, or it's not.