Playboy twins Kristina and Karissa Shannon -- 2 of Hugh Hefner's former 'Girls Next Door' hotties -- were arrested after they allegedly busted each other up.

Law enforcement sources tell us police were summoned to the Shannon twins' WeHo home Wednesday morning around 3:30 AM.

Cops say Kristina and Karissa had gotten into a violent brawl after a night of drinking. We're told Kristina punched Karissa in the face, and Karissa picked up an ADT security monitor and threw it at her sister ... knocking out teeth.

The twins were arrested for domestic violence on the spot, since they both had visible injuries.

Karissa was booked for felony domestic violence, and Kristina for misdemeanor domestic violence. As for the difference ... Kristina's injuries were way more serious.