For anyone still stuck with trying to acquire the final heart in [Together], here's a playthrough video that shows how to solve the last puzzle. I imagine it's difficult to reach the ending of the game if you lose track of your starting location, unless there's something you can do which the developer hasn't revealed to the public yet.

Though flawed, [Together] is still one of the best entries submitted to the Casual Gameplay Design competition this time around, so go try it out if you haven't already before watching this video.