The main idea of this questions is to have two papers under review simultaneously which are somewhat related but do not rely on each other in any way (except in the temporal sequence of discovery for myself). Should the second submission cite the first?
Question 1: Suppose one has a paper under review and are writing a new paper which extends the methods to a different class of objects. Should the paper under review be listed as a reference?
My thought is that if it was someone else's unpublished paper that inspired me to extend the methods, I would definitely cite it, and that tells me I should cite it even though it's my own paper. But it also feels a bit weird too as the papers don't rely on each other in any way really. They are just two different applications of the same proof technique. The applications are somewhat similar though.
Question 2: If I cite it, would I need to include the unpublished manuscript as a supplemental file during submission of the new paper to a journal?
My thought is to not include it, as the work is mostly not relevant, plus it is a much longer and more complicated paper. SO it would make the submission seem very cumbersome. The current paper is much shorter and doesn't rely on the other in any way. Reading the other paper will not help evaluate the current paper. The only thought I have is that an editor/referee might argue that in light of the unpublished work, the new paper isn't a sufficiently interesting new application of the novel proof technique (which was novel in the unpublished paper but now isn't so novel). And that would be a fair point, but I'm not sure if it would be the deciding factor at this particular journal. And I wouldn't want to not include the citation for the sole purpose of avoiding this possibility.
My thought is that both papers are highly relevant, because the results are non-trivial and will almost certainly have other applications that I can't foresee.
These are a math papers, but I imagine the question could be answered more generally.
Edit/note: I believe I am looking for ethical guidance here. I wanted to make sure I was following expected practice of the discipline and what most journals/editors would have wanted in this matter. I.e. if excluding the citation is likely to be seen as unethical, then I definitely want to include it. If it doesn't really matter and opinions are too varied, then I might just exclude it.
I went ahead and submitted the paper excluding the reference though. I'll try to update this if ultimately the reference does end up in the paper though. Hopefully, I get a chance to ask the editors/referees what they desire on this matter.