How to publish two sequential research papers non-chronologically? I am in an engineering field in which conferences are roughly grouped in two types: theoretical conferences with fundamental concepts and related stuff (for instance, algorithms), and application-oriented where people present practical things (e.g. a use-case). These complement each other well, and many researchers attend both.

I am finishing a part of my research which is both fundamental and practical. I have developed a theoretical research result (formulas, algorithms), and I have used it for a practical purpose developing a use-case. Although they are connected, these can be seen as separate work, and two papers can be published out of it.

My problem is that the two conferences of my preference are scheduled in an unfortunate sequence: the practical conference comes three months before the theoretical, and its deadline for submission is also roughly three months earlier.

It means that if I write the practical paper which uses the theoretical knowledge before the one that introduces it and describes it, I cannot refer to the (unpublished) theory, and the subsequent theoretical paper may lose its value. If I tone down the theory and immediately jump to the practical part, the reviewers may ask questions. It's like writing a paper about results of testing a medication, before the medication was introduced.

The field is big enough that I am not worried about a person from the audience asking "Hey, haven't you presented this three months ago on that conference?". I am focusing on how to write the papers in a proper manner.

Is there a way to write these two papers in a non-chronological way?

My current idea is to just tone down the theory in the practical paper (summarise it in one paragraph), and to cite it as unpublished work, or as work under submission (although the paper will not be submitted or even written at that point). And then to extend the story in the theoretical paper which is dedicated only to it.
The description of your discipline sounds like something covered by the arXiv to me. In that case, the best option would be to write both papers in the "right" order and upload them to the arXiv. Include mutual citations. Now for the conference versions, there is no problem anymore: Just cite the arXiv versions.