 The cooperative gameplay originated from players ' requests and from anecdotes of players working together on the same computer or console to solve the game 's puzzles . Wolpaw likened this to players working together on the same computer to solve point @-@ and @-@ click adventure games . The cooperative campaign was also inspired by Valve 's Left 4 Dead cooperative games , in which players enjoyed discussing their personal experiences with the game when they had finished playing it . While the single player campaign in Portal 2 is designed to avoid frustrating the player , the cooperative levels focus on coordination and communication , and Valve recognizes they are much more difficult than the single @-@ player puzzles . Valve did not include timed puzzles in the single @-@ player campaigns in Portal and Portal 2 , but found that their inclusion in the cooperative mode is effective and gives players a positive feeling after they successfully plan and execute difficult maneuvers . Each puzzle chamber in the cooperative mode requires four portals to solve to prevent puzzles being solved by the actions of only one player . As soon as a playtester discovered a way to complete a puzzle with one set of portals , the level was sent back to the designers for further work . Except in a few cases , Valve designed the chambers so that both players would remain in sight of each other to promote communication and cooperation . Some of the puzzle chambers were designed asymmetrically ; one player would manipulate portals and controls to allow the other player to cross the room , emphasizing that the two characters , while working together , are separate entities . The designers soon realized that the ability to tag surfaces with instructional icons for one 's partner was a necessary element , since they found this to be more effective for cooperation than simple , verbal instructions . 
