Can I combine Advantage II and Advantage spray for cat fleas/ticks?

In all likelihood, your kitty will probably be safe. However, I would advise using a product such as Capstar instead (nitenpyram) to most safely kill adult fleas without causing any sort of interaction.

You can use both products, but I do not recommend applying them both at the same time. If needed for additional flea control, you can space out the applications two weeks apart. However, I feel like both of these products have excellent flea protection and 2 different types of topicals are often not needed to control fleas.

If it's been at least two weeks since you applied the Sergeant's, which it has been, it's safe to apply the Advantage now. You will also have to disinfect the environment. Wash their bedding in hot soapy water, and vacuum the furniture and carpet surfaces. You can use a spray to kill fleas in the house, such as Indorex, or a flea bomb would work well to kill the fleas in the home.

I would not recommend repeating treatment with similar ingredients for at least a month, especially in cats which are very sensitive to overdosing. It is important to treat bedding and sleeping areas to kill eggs and larvae before they hatch. If problems persist contact your vet for oral treatment such as comfortis

Hello, yes you can use them together. Most importantly, is that you also treat your environment to rid of the fleas. You can use pet safe environmental insecticides, wash all bedding and clothing with hot water, and replace the vacuum bag after every use. Hope this helps and best of luck.

Cats can be sensitive to flea medications. For that reason it is safest to wait 2 weeks between products even with different ingredients. Your vet can prescribe a cap star which is a pill that will last for a day that can help while you wait the remaining time

Fleas collars do not typically work well. Hartz is not as strong as most other flea medicines and tend to have the most reactions. Advantage is great one that is monthly, Bravecto is another great one and it will last for 3 months but requiers a prescription. I would wait at least 2 weeks in between the topicals for her safety and the flea collar does not matter for timing of products.