Oh, I though you were referring to Adobe Camera Raw. In Photoshop, to show all the open images you need to change the window option to any of the modes that show multiple images:
Menu Windows -> Arrange and select your options (e.g. "Float all in windows")
Then you need to use :
File -> Automate -> Photomerge. There, click on "add open files" to include your open images and you can even add more images.
You have several merge options (layout), but you may want to start with the "Auto" option.