Preventing multi-cat groomers from licking other's flea application site

I put Advantage II flea medication on my two cats last night. One of my cats has a prominent liquid drying spot on his neck still, while the other doesn't have too much of a spot. My problem is that they try to groom each other and I've had to intervene several times to prevent them from licking the back of each others necks.

I'm worried about how this could harm them if they ingest it and how long until it is fully absorbed into their skin. I'm also wondering how I can prevent them from licking on each other in those spots.

Just keep the cats away from each other for 15 minutes or so. Distract the cats with a fishing pole toy if needed. You could also keep the cats in separate rooms until the flea treatment dries and is fully absorbed thorugh the skin.