Because their latest "game" is to hog the toilet when taking a dump so the other one rolls on the floor in agony upon waking waiting for their turn to use the potty first thing upon waking.
No amount of explaining things gets them to the point of understanding that there are other toilets in the house they are welcome to use when their brother is camping out and refusing to get off.
I can just picture how this will progress to locking the door and taking hour long showers while not letting the other one in to get ready for school.
Oh the joys!!!!