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Ignore them. They'll come out of hiding when they're ready.
This. There's nothing to gain by dragging them out of their respective hiding places and trying to force them to be friends. As discussed earlier, don't let them near each other's litterboxes, and try to keep them separated. The procedure on that link I gave you exists for the alleviation of precisely the symptoms that you are observing now. It's too late to start again properly, so all you can do is let them be and hope they don't murder each other before Penny goes home.
Ignore them. They'll come out of hiding when they're ready.