Not a single one of them knew how it happened, and certainly hadn't the faintest clue how it worked for as long as it did, and why it was still working. Then again, none of them really gave a damn, so it didn't matter.
With massive egos and massive holes in their sanity, the most predictable path for Gotham's slowly emerging group of villains (a "rogues gallery" the Internet was starting to call them) was to constantly be at each others' throats, in competition for Gotham City's soul and never too opposed to blowing up someone's hideout or busting their heist.
This was only partly true. Competition of course, that was what made it fun. But the rogues of Gotham were professionals - there was always some method to their madness, if abstract and flexible, making sense to only them. And busting heists was just plain rude. As for being at each others' throats, well...
Jonathan knew there was a word for it, but he'd long since passed the point where he was technical about anything. Harvey didn't think about it, Edward thought about it too much, Harley was having too much fun to think, and Joker just did.
So they shared each other, passed themselves back and forth, a lover for every mood, and somehow it worked.
Joker and Harvey were intertwined long before the other three came into the mix. Their unions were tinged with blood, hate and apathy, but as the years passed Two-Face softened Harvey up, and Rachel became a distant memory as the attorney and clown fit together better than anyone would have believed.
But Harley was their wild card, a firecracker among bombs. She was the newest, but has lasted far longer than anyone had expected. She got back up every time Joker hit her until he finally turned his violence into sex and they became Gotham's most twisted "official" couple. It got easier for Harley to bear, after Joker pushed her out of windows or fucked Harvey in plain sight, after she connected with Arkham's most twisted doctor.
What Jonathan lacked in infamy he made up for in seniority; he had been the first, and had watched the madness trickle in, following his lead before blasting apart the path themselves. As such, he was the one the newbies like Harley and Edward went to, without knowing why. Jonathan knew their minds, and knew how to let Harley grasp at the fading traces of Harleen as she lay in his lap with bruises. Just as he drew little Eddie Nashton in with puzzles he couldn't ignore. The promise of a never ending game led Edward jumping into madness a bit more willingly than he'd like to admit, and the newly dubbed Nygma clicked with Doctor Crane. He was a fun puzzle, they all were.
Arkham's close quarters didn't exactly help, nor did that foretold competitiveness. Fights led to revenge fucks, spats led to new lovers, and soon the five of them were so twisted up in one another that they just stopped caring. When Harvey buried his head in Edward's shoulder and let the Riddler pet his hair, Joker grabbed Jonathan and put a rictus smile on that pretty face. Harley just laughed, bouncing in between her boys when Puddin' was in a Mood. Jonathan and Harvey fucked in cells and offices, and Joker found a very entertaining toy in Edward Nygma.
Alone they are dangerous, but together they become a bloodstained storm of chaos, insanity, fear and unforgiving chance. They were something of legend to the rogues that followed, but by then they'd honed themselves, and didn't let anyone else in to play, no matter how tempting Ivy was or how easily Jonathan and Jervis coexisted. No one could understand why Harvey defended Harley, why Joker didn't kill Edward for his questions, why Jonathan allowed any of them in at all.
But they'd never been very predictable to begin with.