"From half-life.wikia.com:

Half-Life: Decay is an add-on included in the PlayStation 2 port of the first-person shooter game Half-Life, developed by Gearbox Software and published in 2001. A PC version of this game was released September 23, 2008, developed and released by independent Ukrainian developers with the help of other European developers/fans.

Unlike the other narrative-based Half-Life games, which are single player only, Decay is designed for co-operative play. While it can be played by one person, swapping between the roles of Dr. Cross and Dr. Green, the game is intended to be played by two players in split-screen. Various puzzles and combat situations throughout the game require the co-operation of the two characters."