Used to describe Nikki Sixx and Tommy Lee of Motley Crue or Ben Bruce and Danny Worsnop of Asking Alexandria. Similar to the phrase toxic twins applied to Steven Tyler and Joe Perry of Aerosmith.
Two individuals who are best friends or extremely close but cause chaos in each others lives. Usually used to describe two individuals in a rock/metal/punk band, and usually the lead singer and lead guitarist.
Example of terror twins and terror twin behavior:
Ben Bruce once set Danny Worsnop's favorite cowboy boots on fire because he was mad at Danny. They didn't talk for six months until Ben got drunk and confessed, and now they're back to causing trouble together.