#1 means "I agree with you to my maximum amount of agreement. It would be impossible for me to agree with you to a higher degree".
#2 means "I am unable to agree with you any longer. I used to agree with you but now I don't, and I don't think I will be able to agree with you in the future on this matter".
#1 has a more standard form in BrE. If you agree completely with someone, we say "I couldn't agree more".