I use Spot On so I checked what the combo is. It doesn't appear to be stronger but has an added ingredient which seems to sterilise the female flea preventing it from breeding.
If I get the right meaning then regular use of Spot On is fine as there wont be female fleas breeding in the crevices of the house but if you forget the Spot On treatment then it is possible that fleas are in the house and will reinfest the dog.
Eventually by using the combo it will sterilise the females and all fleas in the house die. Plus the use of the Dyson

though.

And a washing machine set at

to boil the bedding and kill the b's and any eggs.
However, in other posts some COL members questioned the effectiveness of Frontline for tick control in some areas of the country so I don't know if that is also the case with fleas. Some others wont use any chemical on their dog.
I can only speak for mine. Poppy had two ticks in spring time then I Frontlined her and Lucy and no more ticks or fleas. But I'm not in sheep country.
What about using a flea spray to zap the blighters though, then the flea comb, then an insecticidal bath as a temporary measure?