That comment from 3.0 and on can be interpreted many ways. One of the many grey areas involving rule interpretations. A strong case can be argued both for and against wearing two magic rings on one hand vs. having to wear one on each hand using it. I will just use my DM's rule, since the DM is the ultimate decision maker.
My neck slot is spoken for already and I do remember wearing an earring as a magic ring in a 2nd edition AD&D Spelljammer campaign. No biggie if there aren't official rules about it anywhere. I will go along with my DM's decision. Just makes the game more interesting.
I know it doesn't seem to state one way or the other having to wear one on each hand. I recently lost an arm and my DM told me he thought you could only wear one on each hand, so was scouring official material trying to dispute that until I got my arm back.