The real problem with joining tanks in the 55g range is the thickness of the glass. even if you Euro brace the top center of the assembly the glass is half as thick as it sould be. The problem is a lot less noticable on larger tanks with thicker walls. That said I once ran into just the sort of situation I read on this thread I found two of the same size tanks...each with one damaged side. In this case it was the front of the tanks. I joined the tanks with an overlaping piece of plexiglass on the bottom and the joined sides. I used one of the existing clear sides as the front. The tank was intended as a free breeder and it was still holding water when I gave it away several years later. The plastic frame was cut..top and bottom and epoxied together. A few of the better glass shops can sell you a can of the adheasive they use to bond pieces of glass together and have them remain clear.
Glass thickness is one of the main differances between aquariums and "reptile" tanks. Stay away from the larger rep tanks for aquarium use.