I'm sometimes a few days late giving my girls their heartworm and flea meds. I give the heartworm meds on the 1st and flea meds on the 15th. I think a few days doesn't matter, my girls have never had a problem.
I don't know if it's a good idea to give them both the same day though. Too much chemical for their systems to deal with at once, that's why I have a couple of weeks in between. And if they were to have some kind of reaction to one of them, how would you know which it was if you give them both on the same day?
Sandy, Koke and Lelaska's Mom