The Co-Op mode in the game is actually a separate mission pack called the Decay Missions.
A shorter, two player version of Half-Life, where you play as two female scientists. The Decay mission pack is split up into seperate missions.
This page contains co-op info for Half Life on the Playstation 2. All information about Half Life was correct at the time of posting. Information is subject to change. If you see any errors please email us.