Today, there's no way to import a project into another.
There are some 'tricks' but you still have to :
-Recreate the layouts with size, options..Etc
-Recreate the all layers with name, fx, options..Etc
-Paste the objects on each layout and position them as the old one
-Recreate the families
-Reimport manually all the sounds
I probably forgot things but what I bump into is enough to come here and ask that question :
"Could it be possible to include in C2 an option that can make that import action easier?'
Even if I already rename every element of the game to avoid duplicates, I really don't know if it's possible technically speaking.
If I'm trying to merge Two projects is because I want to include a minigame into another big game. It's quite common.
It's better to work on the minigame on one side and the big one on the other side. That way I don't risk possible mistakes on the big one and also I don't have to load that big game in memory each time I test something on the little one.
I can also playtest online the minigame easely without sharing the big project.
It could be very useful to have an import project option!