Accidentally saved some content from a cell without having the one I was supposed to save it in as active. So basically I have 50% of my work in one cell, and the other 50% in another, with the same name. How can I combine the content (walls, objects... etc) into the other one so I only have one cell?

Not sure if this would work but you could try selecting everything and copying it then paste it into the other cell then line it up.

Tried, didn't work because there's no copy button You can only duplicate, but not bring it over to another cell, as far as I know.

Just hit Ctrl+C to copy, and Ctrl+V to paste. How could you possibly not know this?

I must have clicked something wrong. Because it just worked. No idea what happened, and yes I tried Ctrl+C CTRL+V
I swear I tried it 3 times

At the risk of necroing a thread... I do this a lot myself... I make a dungeon too big and decide to part a piece of it out to another cell. When I screw it up is because I don't click the render window after selecting all the parts (if I don't do it in the render window to begin with). You can't copy anything until you click the render window, from my experience.