You could create a new document that is bigger than you need it to be. Open both files. Activate one of their windows. Hit ctrl-a (to select all). Hit ctrl-c (copy)Activate the new picture window. Hit ctrl-v (paste). Do the same for the other picture. Click on the layer window to change between layers. Hold ctl down, and drag the pictures into position with the mouse. When you have it the way you want it, use the box lasso command (top left of your toolbar) to draw a frame around the two pictures together. Go to the "image" menu and select crop. Save as jpeg. (in previous versions before 6, it's neccessary to flatten the image, first).
You can also do this by expanding the size of one of the pictures by going to the image menu and select canvas size. Select a direction to expand the canvas and a number of pixels. Cut and paste the second image into this space.
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