Now that you explain it, I remember this behavior of modal.js. But, unfortunately, it doesn't work here, only the Save button is cloned. Looking at the modal.js line 417, $icontents.find(buttonSelector) only returns one button, not two as expected. It seems like the Save+Close button is added too late, after the modal has already cloned the Save button. While debugging, I added the button unconditionally and also removed the uk-hidden class, and then edited the page in a new tab with modal=1, and both buttons were there, as expected. But not in modal. I guess if I add the button directly to the ProcessPageEdit.module (or via hook), it would work. But it's working for you, so there must be something with my installation. It's a bit late here, so I'll continue testing tomorrow.