Old thread, you may already have the answer but I'll help anyway.
I had problems on the level your describing, the games split screen is a glitchy mess and I stopped using it after this level and just played single player. I fixed it by going back to the top floor, then dropping out player 2 and then getting the medallion on my own.
There's no way to fix it that I know off, I advise just not playing the game on splitscreen (or levels which are like this one, small and cramped).
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