Does Exilis or ThermiTight work better for skin laxity on the thighs? Can you do both? Should one be done before the other?

I would say it depends on the degree of laxity.  Thermi tight is a more aggressive treatment with a little more recovery time. Exilis ultra does a very good job of tightening mild to moderate laxity with no downtime.  In our office we use the lutronic infini if a more aggressive approach is needed.  Your provider should be able to advise you as to the best treatment for you to achieve your goals.