I would suspect a bad clunk line in an engine that runs fine through half a tank and then suddenly quits. Either a hole in the line or slipped off the tube. If it's a hassle to get to the tank to check it, you can test the theory by seeing if the engine will run with only a little fuel in the tank.

You also didn't say what kind of planes you were flying. If the engines are in a cowl they may be overheating and shutting down, even if they are tuned right.