Yes, Rockstar announced that because of the release of the Legends and Killers DLC today (Friday 13th August) at 1PM eastern time all games, including co-op and free roam will earn double experience. So those who need that horrible How the West Was Won achievement or the Legendary achievement, this is your chance!
- “Whether you're adventuring the open expanses of Red Dead Redemption's revolutionary Free Roam, teaming up with friends to tackle one of the six unique Co-Operative Missions in the Outlaws to the End Co-Op Mission Pack, or going head-to-head with other players in one of the game's Competitive Modes - including the Legends & Killers Pack's nine new multiplayer maps - all online multiplayer activity this weekend will earn you twice the experience points.”
- — Rockstar
So, pick up your ammo and kill 'em all!