Two papers with identical introduction and related work I wrote a paper (for an economics journal) that contained two main theorems. It was too long - over 60 pages. So I split it to two papers - one for each main theorem (and the related lemmas). Now, the introduction and related-work section in these two papers are almost identical, since the topic is the same. Will this be considered self-plagiarism (e.g. by automatic plagiarism detection tools)? What is a good way to write these papers preventing duplication?
If you treat a different aspect of the topic, it is absolutely fair to split it up in two parts. The introduction will sound quite the same, but this is also the case for most introductions in a specific research field. If you define well the objective of the paper you are presenting, you should be fine. 

Here is an example of a paper (among many) splitted in two parts that takes interest in one main topic : Part 1 and Part 2