Well assuming you have ~2 gallons in each bucket you could buy four one gallon size jugs and split each of your two gallon batches into halves. I buy the 4 liter jugs of Carlo Rossi wine and just dump it and use those usually since they're so cheap at my store (#6 stopper is fine for these). If you're not too keen on the idea of splitting up the batches you'll just have to buy a couple of three gallon carboys and top off with some distilled water.
'Course you could also just mix all the stuff together in a 5gal carboy and top that off with a bit of distilled water as well, but if the batches were split to begin with I'm sure they developed slightly differently even if the same yeast and honey amounts were used since each carboy is its own little world. In the future may I suggest sticking with one, three or five gallon batches so you don't have this problem with odd sizing again? Good luck to you! :cheers:
PS- If you decide to go with the one gallon or 4 liter jugs, the wine thieves which house a hydrometer for easy gravity readings don't fit into the mouth of the jugs. I ordered a smaller glass wine thief for these. The larger auto siphon setups seem to work fine though