With the bare minimal force required to not drop it, Rita
holds a refrigerator magnet — not the lame flexible ones that just stick to a fridge, the good ones that can actually hold papers and photos — up against the mirror.
If there were different universes,
this would be placing the same pole of the magnets against each other, which repel each other; if you've ever done this, you know this repulsion tends to push the magnets sideways relative to each other.
Since this won't happen,
we know Joe's conjecture is false.