I use the borax upstairs on the carpet and the cats have been locked up down stairs where its wooden floor through out. Its the bedrooms that are the problem and not the cats themselves. Because they can not feed on the cats now, the fleas are going for us. lol.
If you would answer the questions it will make it easier for the regulars to help you. I genuinely don't understand - in your first post you say the Borax was effective but there were still fleas in the bedrooms, now you say you have only Boraxed upstairs? Why were you asking abut spot-on if the cats are completely isolated from where you think there are fleas? If you are being bitten then there must be fleas downstairs where you have not Boraxed, in which case there must be fleas on the cats?
I learned the hard way that fleas can live in wooden floors - many years ago I moved into an infested house with only two carpeted areas and caught fleas (saucer of water) on areas well away from the carpets. Believe me vacuuming and hot washing bedding made no difference at all - we had the professionals in three times.
You will be transporting flea eggs and borax around the house on your feet and clothing, I just had to Stronghold my indoor cat Noah after I brought something into the flat.