When the window is at maximum (50%) open position, looking from inside, the right pane is slid left so it covers the left pane. Have you not watched the video demonstration in Amaral's film?
I know the window slides open from the right and covers the left but why open that curtain when it was trapped. You only need to slide open one curtain to go though the open window side or to look inside the room. Why open the other trapped curtain on the closed window side? Gerry said he closed the window (possibly destroying any intruder fingerprints on the handle!) not Kate and the fact is that looking through the open window is missing from her statement.
"At around 10pm, the interviewee went to check on the children. She went into the apartment by the side door, which was closed but not locked, as she said before. She noticed that the door to her children's bedroom was completely open, the window was also open, the shutters raised and the curtains open, while she was certain of having closed them all as she always did.
Faced with this situation,she verified that the twins were in their respective beds, unlike Madeleine, who had disappeared. The cover was pulled back and the toys were on the pillow as usual. After searching the whole apartment thoroughly, the interviewee went back, scared and shocked, to the restaurant, to alert her husband and the others to the disappearance."
http://www.mccannpjfiles.co.uk/PJ/KATE-MCCANN.htmOne curtain is trapped between the bed and wall.