It will kill adult fleas in 24-48 hours. If you are going to get rid of fleas, you need to treat EVERYONE for at least 3-4 months. That means frontline once every 30 days.
Fleas lay their eggs in the carpet, so it doesn't matter if your cats come in direct contact with one another. If their are fleas on your cat, they are in your carpets and you MUST treat everyone for 3-4 months. Also vacuum often and throw away bags immediately so the fleas don't crawl back out.