I love Comfortis! I use it for my dog....
Here is what one vet suggests... I don't know how big your dogs are, but he suggests to his clients to buy the NEXT bigger size, and break it in half...
(I know that Comfortis doesn't recommend doing this... but I was at a standard physician's office recently (in Las Vegas), and they had a poster in one of the exam rooms talking about they will write a script for a larger dose and have you split it..... if physicians and vets say it is ok, I don't see a problem... just split it carefully.)
For example, my dog is about 55 lbs... so I buy the 60-120 size, and break it in half.... that way, instead of the $15 each (from one vet, another charges $18 each), it only costs $7.50 a month... still expensive, I know... I sure wish they had that for cats!
My daughter's MIL has several small dogs... so she buys the 60-120 and quarters it for her little dogs (probably 10-15 lbs each) ... that way she gets 4 doses out of one tablet. She goes to the same vet I went to.....