Justin Bieber & Chris Brown -- Probation Turn Up in the Club

Renegade pop star Justin Bieber and outlaw R&B star Chris Brown joined forces Tuesday night on the Sunset strip and there were NO injuries ... if you discount the bystanders who had to witness their endless posing with their posses.

Breezy and the Biebs -- who are both on probation now -- were inside Bootsy Bellows nightclub throwing up non gang-related hand signs. Shirtless (as usual) Justin went with the classic westside ... while Chris opted for the tried and true bird.

Chris brought gf Karreuche Tran along for the night ... which ended without any fights, broken glass, eggings, or arrests. Hell, Chris even walked out on his own two feet.

Win, win.