Split paper into two, one was accepted, is it fine to negotiate the rejected paper to be accepted? I had a paper ready for journal publication, but decided to break it into 2 conference papers and submitted them to a conference.
Now, one of them is accepted but the other one is rejected! The thing is, the 2 papers are related and I think they should be published together. The paper which is rejected, had 3 reviewers. One accepted it, one weak-accepted and the other one rejected it. The two who accepted it, marked it as innovative, highly useful and relevant. (All 3 reviewers mentioned they are highly confident on being able to judge my paper)
Now, can I write to the conference and tell them to review the paper or argue with them that the paper could be accepted based on points made by the two reviewers?
PS: I had to break them into two papers because of the page limit. Furthermore, it makes more sense for them to be published together since they are parts of a single framework. However, this doesn't mean that one cannot be published without the other; as I mentioned, I believe publishing them together just makes more sense. Moreover, I was very confident that both will be accepted!
Now, can I write to the conference and tell them to review the paper or argue with them that the paper could be accepted based on points made by the two reviewers?

You can try, but what do you have to tell them that they do not already know? They have already read the reviews, thought about them and reached a decision. Unless you can think of a compelling argument that invalidates or rebuts the criticism of the unfavorable review, your chances of success are essentially zero.