Can you mix CoolSculpting and Kybella to eliminate fat under the chin area?

This is a reasonable approach - to use Kybella for the "clean-up" areas remaining after Coolsculpting.  Alternatively, a little liposuction under local anesthesia could be performed.

You can definitely alternate or switch between these two treatments to get optimal results.  Probably not the type of treatments that you would want to combine in one visit but alternating between these treatments is no problem and will likely yield great results.  Best of luck!