What do you mean by "agreement"? Endorsement of a shared verbal description of some assertion about the topic? Complete agreement about the probability distribution among hypotheses? The wording here suggests the latter. For flawless reasoners agreement should always be possible in principle, but may require the sharing of literally every piece of information available, including everything they've ever experienced and the complete structure of their brains, which is not something humans are physically capable of sharing (let alone processing).
In other words, when you say conversations can be long, are you thinking about how they can very easily become millennia-per-discussion-point complicated if you really seek complete agreement? If that's your goal in reasoning and your criteria for success, then yes, you will usually fail. It is not my goal.
Also, to the title question, reasoning does not require a conversation partner at all. Having one can make things easier, or sometimes harder, but not having one does not learning or reasoning.
Because it looks like you haven't explored the assumptions you make well.
If I have a discussion about poker and the person who I ask something about poker has no interests to continue to talk to me about poker I have a variety of ways to increase my knowledge about poker. I might read a book about poker.
In your last question, someone told you to read the sequences. If you want to understand how discourse is seen on LessWrong that was a good suggestion.
I must admit that I'm a fan of this video:
I understand that there are topics that you are interested, and there are not. I, for example, am currently interested with this topic and ignore the rest of others in the homepage.
If you decide to join this conversation, I guess it's safe to say that the topic stimulates the interest of yours than other topics. So we share a mutual interest. Let's say we start talking back and forth, and then you lose the interest of it while we are still not completely agree in all aspect, then how can the goal of reasoning be achieved? I understand that our conversations can be long, and we all need rest, but I suppose we our minds should keep our interest on it until we share an agreement. That's how I take on the theory of reasoning in the video. I guess if one of use lose the interest before an agreement comes, then the purpose of reasoning fails, and that's bad for both of us.
I can end this question by asking what you think about this, and I will. What do you think about this? But I guess the most valuable answers are not from the ones whose interest are still fresh, but the ones who have lost the interest and about to abandon it. What makes your interest lost? Do you think the reasoning goal has been fulfilled?