Maybe make an 8 man dungeon that separates into two and have them both clear different objectives?

Otherwise, that dungeon design will only lead to people finding out the least effort path, nullifying the reason for "dead ends" and other ways to get to the finish line.

In mazes (dungeons), the first and foremost objective is to get to the finish line. Other optional routes to other destinations are considered just that, Optional. So either make those "dead ends" reward something unique or the extra rooms and routes would be just for nothing.