Cave Johnson and Doug Rattman really could secretly be the same person. Okay, it probably isn't true. Or is it? It answers most of and/or all of the mysteries in Portal and Portal 2. Here are some.
1. Why did they put Caroline in cryo-freeze to prevent her death but they did not do the same for Cave? They did. Secretly. Over-suspension is what made him insane, and that insanity is what made him forget he was Cave and think he was Doug Rattman.

2. How did Ratman survive the neurotoxin? He was frozen.

3. How does Ratman know Cave Johnson's login and password? He must have kept the memory through cryo-freeze, unlike his name.

4. Where did all those mugs come from in the famous "room with the hidden stairs"?
He must have realized he was sick, but without knowing he had kidney failure and imminent death, he was testing liquid cures for all known curable diseases.

5. Was it Ratman or was it a Frankenturret that ran past at the turret opera? It was both. He must have realized his death was imminent (again) and, the consciousness uploadable computer already being perfected, turned himself into a turret (Which could possibly have been the Oracle Turret). Which was turned into a frankenturret. Which ran past at the turret opera.

It is still unlikely, but if they aren't the same person, then how can Caroline be Chell's mother if Ratman is Chell's father and Caroline obviously "likes" Cave Johnson. They are the same person. It's just logic.

"I think that neither Caroline or Ratman are in anyway related to Chell. Cave Johnson is definitley not Ratman. If you took the time to read the Portal 2 comic, you would see how the events unfold, along with who Ratman was and his job in Aperture. He chose Chell not due to his relation to her, but because of her high tenacity see Portal 2 comic. It is implied that Cave is dead, and Ratman is suffering from paranoid schizophrenia. Again, if you took the time to research any of this, you would have facepalmed yourself. I hope that since you wrote this post, that you've read the comic, and realized the stupidity of your comments.

Also, Ratman was never frozen, until after he got shot comic pg. 27, and the comic explains how he escaped the neurotoxin on page 20. Do us all a big favor, and READ...even Wheatley would before saying such things."

 A big Problem in your theory is, that Cave is way older then Ratman. Cave was at aperture since the beginning and made fixtures and shower curtains before.
 In some older chambers where you can hear his memographics he is seriously ill.

 He didn't survive this (afaik). If it really was Cave and he somehow survived, then he must be ~60-80 years old if not older. Doug Rattman is portrayed as a young to medium-aged male. In the old chambers are images of Cave, portraying him as an old man. I'd say that Rattman is MAX 40 years old. This doesn't quite add up.
 While Rattman COULD be a clone with Cave Johnson's mind, I doubt this. Doug looks nothing like Cave. Plastic surgery could be involved but this is a far too vague guess.

I think Cave and Caroline are Chells Parents. But a kid would be in the way at work. So my guess is that Cave forced one of his employees to adopt her, and that employee is Rattmann. Proof of Caroline being her mom is the sentence "She was alot like you" in the ending song, 'she' is refering to Caroline. I guess the combustable lemon doesn't fall far from the tree.