It’s possible!
The best way is to open up the bigger of the two images, and then use the File ▸ Add Images… menu item to add your second image to the canvas. Then increase the canvas size via the Image ▸ Resize Canvas… menu, and double the width. Move your second image over to the right and get it all lined up. You can then use the Image ▸ Trim to Edges… menu item to get rid of any extra space that might be around.
Hope that helps,
-gus