In Preferences/Interface uncheck "open as tabs".
So you want two images on one canvas?
You can create a blank file (Ctrl N) and drag (Place) both images into it. You would use the Place handles to resize and re-arrange the images or Ctrl t (transform) to do any further adjustments.
You can "open two images at once". Ctrl O for the Open dialog box, Ctrl + click to select the two images, then click Open. But using that method it would only open them as separate documents.