You turn the 7 minute one and the 11 minutes one at the same time. When the 7 one finishes, turn it again. When the 11 one finishes, you know that 4 minutes have passed, with 4 minutes to get to the desired 15, all you need to do now is turn the 7 minute hourglass again since it counted 4 minutes, and when it runs out it'll be exactly 15 minutes.
Seasonal Guard
Riddle:
One winter day, there was a man standing in the middle of someone’s front yard. The person stayed there for several weeks without moving and the owner of the yard didn’t mind. Eventually, the man left. Who was he?