I don’t know if this is what you’re asking but perhaps: Try toggling between 2d and 3d views or minimizing whichever window you don’t want to see by clicking on the minus (-) sign in the right hand corner of that window. It will still be there but just underneath the other view.
Both windows are always there and represent the same document, so you can’t really ‘close’ one without closing both and therefore the document.
The close button on the MDI//child window probably shouldn’t be there.
One solution is to “making it look like it’s closed” is to maximize one of the windows and then use the 2d/3d toggle buttons when you want to concentrate on one view at a time…