Docs should be where the users are looking for them. The preferences (how/where do users search and therefore, where do they end up) of quick docs users and long docs users could be different (that is not an argument, but an open question that we have to evaluate). If these preferences are different, splitting the docs may increase (the respective) discoverability. But if the preferences are comparable, splitting docs may indeed decrease discoverability.
On one hand, I would assume that quick docs users are more likely to find what they seek on ask.fedora. Many end up at ask.fedora, which we see in the many threads/questions we have. At the same time, I would assume that most “average users”, which are mostly interested in quick docs, do not know about docs.fedoraproject.org. It is easy to get there when I know the url - but I have to know it: I just tried to get to the docs by starting at fedoraproject.org → I get the Fedora editions and spins, and learn how to join the community, but no way that an average user will find docs that way (maybe it makes sense to also talk about adding docs more obviously on the main page? In the support section on the bottom at least? ask.fp is already there, but not docs).
Therefore, this links to the other issue we already discuss: Search Engine Optimization. Most users use at first their preferred search engine, making SEO a big issue in terms of discoverability. I think search engines are currently the major path to docs.fedoraproject.org.
Thus, does splitting the Docs negatively impact SEO rankings? I think that potential issue has to be considered.
I think we first have to identify: where/how do users search long docs, and where/how do users search quick docs? This may start with first asking which users use quick docs and which users use long docs (“which users” mostly in terms of knowledge/experience level: this determines how and where they look for something / formulate queries). Of course, in all that, we have to keep the potential SEO issue in mind (and maybe talk about making docs more obvious on pages that users tend to visit, to less rely on search engines).
A very probabilistic & experimental thing in my opinion 
But I tend to give the Docs plugin at ask.fp a try. It might also bring the teams closer together and foster integration.