This is my own personal theory that I'm almost positive is true.

In one of the 'Wheatley Chambers' that you go through, Wheatley struggles to tell you that he was making a test for you and nobody else. Then he notices that the test has no exit so he brings over a different one. In the process, if you look to the right you can see P-Body walk out of the door, look at you, and walk back in. With the information that I have, I believe that Wheatley's original idea was to make the test for the co-op bots.