For one reason or another, some of us still use both X-Plane 10 and 11 (perhaps the aircraft you love doesn't exist in XP 11 yet or you're not ready to fully transition but there's this amazing aircraft in 11 but not in 10).
Whatever your reason might be, having the same scenery on both versions can be a nightmare or plain impossible due to the amount of hard disk space required. Fortunately, there's a very easy way of solving this issue.
I'd like to start by thanking our good friend Bel Geode for this tip. It's quite a simple and quick solution for a problem that has other solutions but they require you to do some stuff that you might not be comfortable with (like a symbolic link).
Bel Geode's solution (it probably isn't actually his solution but let's pretend it is for a second) offers you the same result. But using something you already know how to: copy and pasting. But with a twitch.
If you have both sims and you have the same scenery installed on both, you probably copied the folders from one version (let's say 10) and paste them into the other (11).
With this solution you still select and copy the folders you want to pass on to the second version of the sim.
The thing is that you do not paste the folders. Instead you paste a shortcut to the X-Plane version you want the scenery in.
That's right: you don't physically copy the files but X-Plane still recognizes it.
If you wish to reorganize your scenery, delete the scenery_packs.ini file inside your Custom Scenery folder and run X-Plane. Your scenery should now appear on this version of X-Plane as well.
Thanks to Bel Geode for the awesome tip and for Laminar Research for allowing this.
Now I have to say that I tried this with aircraft as well but, unfortunately, it does not work, which is a real shame. It's good enough that we don't have to duplicate scenery though. Great stuff, isn't it?