Meh, this was a rookie question. I use one layout for every level in my game. I think I asked this question because I wanted split-screen players to be in different levels at once, allowing them to team up or play independently at will.
To achieve this, I created a system that loads two levels into the layout, splitting the layout halfway, so one half is for one player and the other half is for the other player. The levels load into those areas. It works well, besides, I'm also making a full editor so that iterating on the game will take an incredibly short amount of time, at least from a development standpoint.
I didn't want to do this originally, because two years ago I wasn't actually able to. So, I guess it all comes down to: If you want a feature that isn't really natively possible (at least not easily) in Construct 2, build your own engine inside of Construct 2. The editor I'm making has a cutscene/dialogue editor with branching paths, as well as a tilemap/terrain sculpting/plotting interface, and visual scripting. I'm using Construct 2 to "build the bones", so to speak, but when it's complete, it will likely feel more like an independent engine.
Also, if you're wondering about performance, it's not an issue. I've created a lot of different methods to optimize processing and memory use that I will likely make tutorials over when my game is complete. As well as tuts for everything else.