Only once have I seen 2 chickens in a box, but it's big enough they were next to each other, I keep expecting to see someone sitting on another but I haven't come across that yet! The older one that is no longer laying will use the other box if she absolutely has to, but no one else will - they just also do the pee pee dance. It's so dramatic to be a chicken sometimes.
Top 3 boxes are very popular. Here, two on the left, side by side.
But, on another day - one on top of another. Another day, there was a 3rd chicken stuffed in there too, but half hanging out. Of course, no phone with me that time...but guess there was some pecking order conflicts that day.