I used to always finish my current project before I start a new one in order to get it done.

But now I really like to work on multiple projects, switching when I feel like it. I find after long breaks you see your game with a fresh set of eyes and flaws become more obvious. Also there is less fatigue and boredom, especially when two projects are very different from another. (Currently working on a futuristic mech game and a primitive forest survival game)

Short gamejams also help taking a break from a big main project, but I'd like to know if others also have more than one larger project in the works, and how they feel about that.

Yeah I do this. I think it helps a lot, especially when there is a game design idea I am itching to try out that isn't compatible with my main project. I love spinning up new projects to take a break from the main

Nothing beyond the initiation stage of development. 

In other words, just concepts. It doesn't sound safe to be actively working on multiple projects without securing that one meets certain standards.