1. Load both images to the working area under edit. Make sure they are not tabbed. (Window - untick Tabbed Documents if it is ticked)
2. Select each image( click on its title bar or in the image with the pan tool =hand selected) in turn and roll the mouse wheel so they are fitted to the window. Note down the width and height of each.
3. Add the two widths together eg 200 + 300 pixels = 500 pixels and add 20 pixels = 520
4. Add 20 to the maximum height eg one is 450 pixels high the other is 460 so choose 480 .
5 . File -> New New Image dialog box opens . Type in for my example width 520 and height 480.
6. You now have 3 images open on your work space. Blank Canvas , Image A (front of figure) , Image B (back of figure).
7. Right click the image (A or B) you want on the left and choose Copy. Right click on the canvas and select Paste as New Layer. That image should now appear in the middle of the canvas.
8. Repeat for the second image (B or A) . That image should cover the first one on the canvas.
9. Now choose the move tool - second from the top. Just below the hand/ pan tool . May be hidden and is a 4 direction arrow.
10 On the canvas click on the visible image and drag it to the right and as you do so the hidden image should become visible. When that first image is placed as you would like it drag the visible portion of the other image to the left and position it.
11. Up on the Menu bar choose Layers -> Merge All (flatten). Crop what you want (5th tool down - Crop Tool ) and save the new combined image.
That is just one way and saves you even having to open the layers palette if you are not familiar with that.
12. Similar method can be used to place 3 4 or more images together.