Just adopted a cat with fleas. Can I use Advantage II after Hartz?

Congratulations on adopting Butterscotch! She's adorable! Good for you for having Advantage II on hand - this is a much more effective and safe medication than Hartz flea treatments. However, I generally advise waiting at least 2 weeks between applications of topical flea medications. Fortunately, there are a few other options for treatment at this point. The ideal option would be giving Butterscotch prescription-based Comfortis, which is an oral flea medication that works quickly and effectively, and can be given at the same time as topical medications due to different mechanisms of action. However, this may require a trip to the vet since Comfortis is prescription based. The other option would be to give Butterscotch a dose of oral Capstar (available through most pet stores) which will quickly kill adult fleas, but will not provide lasting prevention. After a dose of Capstar, you can also give her a bath in original Dawn dish soap to kill any lingering adult fleas and remove any flea dirt with the hopes of keeping the fleas at bay for the next week until you can apply Advantage II. Be sure to keep both of your cats (and any other pets) on good quality flea prevention regularly for at least the next few months. Also be aware that cats can acquire tapeworms from eating fleas infected with this parasite, so it may be worth discussing a deworming treatment with a praziquantel-based medication with Butterscotch's vet. I hope this helps and that all goes well!