This is a two-part question, and I hope that's okay.
1. Would it classify as a sequel
I've created a 'universe' in which one main story takes place so far. This will presumably be my first book. It would classify as a sci-fi universe and it is mostly ours (on earth) with a slight twist. The twist could in fact simply be unknown to most people on the planet.
While writing the first book - still underways as I write this - I've gotten an idea for another story in the same universe.
These two books would share the slight twist which makes the universe unique, but the stories share no characters, locations or plots, so far (The second book is only sketched out a bit), and I doubt they will. It would, in theory, be possible for some characters to reappear in the second, but there doesn't seem to be a need for it.
The second book would explore other sides of the universe and have the characters go through different quests, dilemmas and so on.
So, would the second book classify as a sequel from the description I've given so far, or would it be something else?
2. Would a shared title be appropriate
So far I'm counting on having a shared title and a sub-title like this
The Universe - Title of /the focus of book 1 -
The Universe - Title of /the focus of book 2 -
I'm unsure about whether this would be helpful or confusing to readers or fans of the works. If it makes it seem like a sequel even if it isn't, I would probably change that.
I guess an example comes to mind that is comparable; The Hobbit vs LOTR.