I know what is the meaning of "I can't agree with you more", but what about "I can't agree with you enough"? What does it mean? Does it mean that I agree with you, or I don't?
Firstly, to answer your question, both phrases are used to convey essentially the same meaning, as they are different approaches to conveying the highest level of agreement.
"I can't agree with you more" means that one's sentiment of agreement is as high as it can go.
"I can't agree with you enough" means that verbal expressions of agreement are insufficient to convey one's sentiment of agreement.
I do not agree with Charlie Brown though, that one is "more proper" than the other. I would speculate that "more" is more widely used in US English, and "enough" in UK English, in a similar way to the US "I could care less" and the UK "I couldn't care less".
"I couldn't agree with you more" and "I can't agree with you enough" are two different ways of saying what is intended to be the same meaning. For instance, Person A makes a statement to which Person B replies by using either expression to convey that they feel the exact same way about Person A's statement. Both expressions are in a sense an exaggeration to drive home the fact they are in complete agreement with what the person's thought or feeling.