Put the dummy eggs under the hen that isn't laying at the same time the other pair lays. When the eggs hatch, give the babies to the pair with the dummy eggs. ACTUALLY, what I would do is just give them ONE baby.....if they take care of it fine, then you'll know next time. If for some reason they DON'T take care of it, you can just put the one baby back under it's real parents with it's brother/sister and no one is the wiser.
If you take both babies and then, say, 4 or 5 days later, they decide they don't want to take care of them, then most likely the real parents will have moved on and won't except them either......then guess who MOMMY will be?? LOL