1 - open the first photo in PSE and go to the Image->Resize->Canvas Size menu option.
2 - change the width measurements to percentage and enter 200 and hit Enter
3 - When you're back to the enlarged photo canvas go to the Layers pane and double click on the one layer to unlock it.
4 - Now, with the cursor in the Move Tool mode, Shift-Left Arrow till the image moves to the far left of the canvas. Releasing the Shift will move the image in smaller increments.
5 - now open the other image and drag it into the first images window and close the second image.
6 - move the second image to the right and position it where you want.
7 - do a Save As to the desktop to create a new file.
That's it. You can modify the above to place the photos exactly where you want, left or right, etc., but that's the crux of it. Be sure to do a Save As so as not to modify the original photo files. Good luck.