I found this old thread because for the third day in a row, my Silver-Laced Wyandotte laid two eggs within minutes of each other. She is the first hen to lay in our flock of young hens (she was just over 18 weeks with the first egg) and I thought at first that we must have a second layer. So today, I kept a close watch. I did see one of our White Rocks in a nest box, right above my Wyandotte, so I thought it was her. But when I went to check later, the Rock had laid her first egg, and once again, the Wyandotte had left behind two in the box she'd been laying in, for a total of three eggs today. When I saw this post and read that another Silver-Laced Wyandotte laid two eggs, I'm thinking it must not be uncommon for this bird.
So, it IS possible for a hen to lay two eggs in a day. And my hen lays them in the same sitting, within minutes of each other. And if you'd like to see this for yourself, get a Silver-Laced Wyandotte--a great laying hen who is also beautiful.