I have 2 small dogs (10 lbs each); can I split a flea medicine pill made for a larger dog between the two?

I know theoretically this would work, but I love my dogs and want to make sure this would be safe. I want to start giving them Sentinel for heartworm and fleas, but it is a little more expensive. To save money would I be able to buy the Sentinel for 26-50 lb dogs, cut it in half, and give a half to each of my 10 lb. dogs? Please don't speculate. Any veterinarians out there who can tell me for sure?