To attach two branches together, you'd really want to use either a mortice and tenon joint, or a reinforcing dowel through the center of both branches. The later is easier. You plane, sand, or file each end flat. Then drill a hole in each end. Then you insert a dowel, about 4'' long that tightly fits into each hole along with some epoxy or gorilla glue.
Things like lacquer or resin aren't going to do much to strengthen a branch. At most, they will prevent the bark from getting scratches...and who cares if you scratch bark? To really strengthen it, you'd need to do something wild, like wrap it in fiberglass cloth (which would make it look pretty darned unnatural), or split the whole branch up the middle, core it out, insert a steel rod, and glue it back up together.