An idea of what your nests look like in relation to each other and what you are using for fake eggs might help us as to how they got together. It may indicate you need to modify your nests some.
But for your real question, when will it happen? There are different signs that they may be getting close. But these are signs they might be, not absolute proof. Combs and wattles turn bright red, the vent may become soft and most as opposed to tight and dry. Sometimes they squat for you, though I don't put much faith in that one. I've seen plenty of exceptions to the squatting, but it is still a sign they might be.
But often pullets start looking for a good nest maybe as much as a week before they start. Not all do this, some seem to have no control over their first few eggs and drop them wherever they are, on the roosts or just walking around. But many do have that control and want a safe place to go when it is time. That search often involves scratching. Hopefully this is involved in what you are seeing.
