Well... as usual the answer is annoyingly simple.
All you have to do is shrink "the key" horizontally until it's an invisible line. It still functions normally even though you can't see it in the editor anymore (drawing a box around it will allow you to select and resize it if you need to).
I spent a good half an hour with a screen literally filled with treasure chests and try as I might, I couldn't get two of them to trigger at the same time.
So... success! Thank you very much once more, Jeswen and TheInstance. You two were both a huge help in getting me started with my project.