An open relationship

I said "I know, they came out of the closet this morning".

Credit to u/echonight . This is a cross post from r/askreddit

There are two identical twin brothers that live together. One happens to be a well-respected dentist, and the other can't seem to keep a job. Instead of actively looking for work, he likes to sit around at home. One Saturday, the dentist is hungry, and puts his brother on the spot. He tells him to get off his lazy behind and go get them some food. After some protest, the lazy brother takes the car and leaves for the store. In the meantime, the dentist takes a nap on his day off. He turns off his phone so he won't be interrupted.

About 30 minutes later, the lazy brother gets into a head-on collision in the intersection by the grocery store. His vital signs are fading; he's unconscious and barely moving. An ambulance picks him up and rushes him to the hospital. He ends up in the Emergency Room under observation, but his condition is critical. They try calling his dentist brother, but he doesn't pick up because his phone is off.

The dentist wakes to a knock on the door. Suspecting a solicitor, he ignores it, but the knocking continues. Eventually, he resolves to get up and yell at the person at the door. When he does, he reveals--- the grim reaper. He is just as he appears in movies; a full skeleton underneath a tattered cloak.

The grim reaper swears. "Oh no! This always happens with identical twins".

"What do you mean?" asks the dentist.

"Well... if you must know, your brother was in a critical car accident, and I've come to take him to the underworld. I'm afraid his time on Earth has ended. I'll take my leave now."

The dentist is noticeably upset. He says "Wait! Isn't there some way I can challenge you for my brother's life? After all, YOU made the mistake. Certainly there must be a way I can bargain for his life."

The grim reaper asks "What do you have in mind?"

The dentist thinks. "How about a challenge? If I beat you, you let my brother go free."

The grim reaper laughs. "I will beat you in any challenge. What challenge do you propose?"

The dentist smiles. "I propose we see who has the cleanest teeth. 5 minute of brushing each, then we decide."

"Very well" says the grim reaper, who makes his way to the bathroom.

Once there, he pulls back his tattered cloak to reveal his skull. It's glistening. He takes a toothbrush from the bathroom, loads it with toothpaste, and brushes. After 5 minutes, the shiniest teeth anyone has ever seen glisten and make the room bright. The grim reaper gr