Kybella can melt the submental fat and cause some tightening. Usually I prefer to see the effect of the kybella first as it may change how much filler will be needed in the jowls. So to answer your question, I recommend to do them weeks apart.
Since you are talking about treating two different areas, no, there should not be a problem getting both Kybella and filler to the jowls at the same time. Make sure to see a board certified plastic surgeon or cosmetic dermatologist with a lot of filler experience.