Say you're working on two papers A and B simultaneously, and the content and topic of the papers partially overlap such that normally you would be required to cite paper A in paper B. But since both papers are being written simultaneously how do prevent self-plagiarism?
Should you cite paper B in paper A and vice versa, even though neither is published yet?
Yes, I would cite paper A in paper B, and I would cite paper B in paper A. Initially the citation would be something like “in submission” with a URL for the referees. As both papers work through the pipeline, that citation would evolve into either a refereed publication venue if possible, or a preprint server if not, to ensure that future readers can follow the link.
On the other hand, I would also write both papers from scratch, without sharing text between them, because (in my experience) that leads to better-written papers. So self-plagiarism isn’t an issue.
With respect to accurate accountability: you are required to cite anything taken verbatim from another source, regardless of where, when, or if published. If we're not talking about importing exact language and you're referring instead to ideas you've included that are also present or explored in your other publication, it works as follows:
Your paper that is published first is free from obligation. Take whatever you want from your other paper's drafts and set it into print.
Your paper that is published second must cite properly any elements that were originally published elsewhere.
These two guidelines hold true down to the nanosecond. If both publishers produce on the same night, you still need only worry about accountability for the second one. But, since that's really a ridiculous pain in the ass, why not just cite everything (using a label for "pending" in your references)? Life would be easier that way.
It depends on whether these are the same type of paper. If they have different focal points but share certain informative paragraphs introducing a certain matter, it makes sense that they would be similar. If these are opinion papers with overlapping ideas, that could count as a form of parallel submission, though I doubt this is the case.
This is really a matter of personal consideration. I know theoretical biologists who repeat certain paragraphs in almost every paper they write, with some variation. So long as it isn't blatant verbatim copying, its only a question of whether this copying reduces the significance of one of the papers in the eyes of the journal. If it doesn't then I wouldn't worry about it, especially if the information is necessary to provide background in both papers.
