Is it safe to use both Sentinel and Frontline together for my dog's flea protection?

I understand that Sentinel is for flea & heartworm protection.  I also understand that Sentinel only prevents the development of flea eggs, but does not kill adult fleas.  Frontline, however, both prevents the development of flea eggs and kills adult fleas.  Probably the best solution is to use Frontline for fleas/ticks and Interceptor (which is the worm prevention half of Sentinel) for worms.  However, I already have a year's supply of Sentinel and my dog has picked up adult fleas.  So I would like to give him Frontline to kill the adult fleas while he remains on Sentinel.  Is it safe to mix these two drugs?

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I have 4 dogs and a cat, every vet visit costs me over $1000. I recently ask my vet if Sentinel was better than Frontline when it came to flea protection and she advised using the Sentinel for the cat and two seperate products one for fleas and one for heartworms for the dogs. Using both on any animal, cat or dog is dangerous to both the pet and the owner because of too much flea medication being applied to the pet.

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