It's these two pressure plates located in western portion of Tianqiu Valley here:
I've spent a bunch of time using Geo MC, trying a combination using two rocks on both plates, one rock on one plate and another character with Amber's Baron Bunny on another plate, and nothing has worked. Both plates make the audio cue that they are both activated, but no chests show up. Are there even chests that show up here? Or is this puzzle meant for something else?