A single file would contain both your paper and slides (via slide tags).
You could even have a slides view that you can switch to while editing your paper.
Slide tags would allow you to generate slides for a paper without heuristics. The result would be completely predictable.
The slides would also be more maintainable since they can share objects with the paper so a change in one will be automatically reflected in the other.
Nice idea indeed.
In you idea, will the slide-breaking management be at the user discretion or achieved automatically ?
The latex beamer class as something similar but the opposite way with a beamerarticle package that typesets the contents of the beamer presentation in an article layout (see section on Creating Handouts Using the Article Mode in the Beamer doc).
I don’t know how slide management normally works in TeXmacs. But the general idea is that you would have at least as much control with this approach. Tags would be used instead of heuristics.