Ironically, Gun Bros up until this point had no co-op play despite its name. You see, instead you imported AI copies of your friends to help you along your way. Now the iPhone/iPod game - as the developer Glu Games points out - it FINALLY has realtime co-op play.
Two players can play via Wi-Fi or 3G connections and enjoy daily Bro-Op challenges to earn in game cash. The free to play game utilizes an in game cash system with microtransactions, but these Bro-Op challenges lessen the need to open your wallet. Perhaps the coolest feature in addition to co-op is real time voice chat.
The update also tweaked some of the core game mechanics, fixing some auto aim issues and adding improvements to the audio. Gun Bros is out now and it's free to play.