It is normal. Chickens do that quite a bit. For whatever reason, they sometimes get it in their minds that one box is somehow "better" than another. It's not a bad thing really. They will eventually sort that out amongst themselves...or they won't and will continue to share the one box.
The pictures of the hens sharing the nest boxes are great.
I have 2 broodies in 1 box. The 2 times Ive gone to candle, the eggs are all under 1 hen. Not sure what will happen when they hatch this weekend. 2 mommies I guess.