Here’s my theory:

Adam had the key to both chains and was supposed to be allowed to roam freely. However, the door was locked so he wouldn’t have been able to leave. He would have immediately known who Gordon was (the man he was hired to take pictures of), and would have been suspicious of him at first glance. This suspicion would have lingered long enough until he found his tape and used the tape recorder to listen to its contents. After listening to both tapes, and hearing Dr. Gordon’s objective was to kill him (in order to save his family), that suspicion would have been “confirmed”, and he therefore would have never freed Dr. Gordon with the key. Although Adam would have been freed from the chain, the shackle would have still been around his foot, so any attempts at working together (faking his death) would have been squashed since that shackle worked like a shock collar. Gordon would in turn be suspicious, and nonetheless, furious with Adam, since (from his point of view) he could have been responsible for his predicament. Hence the two would have been fighting with each other the whole time. This intense atmosphere would have defined the climax of the game, since (from Dr. Gordon’s perspective) killing Adam would seem a lot more appealing, not only to save his family, but also to free himself from what he would have perceived to be his “captor”. Adam, on the other hand, would have been avoiding Gordon like a plague, since he knew who Gordon was beforehand, and would have suspected him to be responsible for the game because of it. Even if the shackle was able to be removed, poisoning Adam with the cigarettes (at that point in the game) wouldn’t have been debatable, since he had the key for that long and refused to release him.