Basic photo editing on OS X
You can do this simply with Preview. Open the first picture. 


Choose Tools > Rectangular Selection.
Select the whole picture.
Copy the selection (choose Edit > Copy).
Choose Tools > Adjust Size.
Turn off the lock for proportional changes.
Select pixels as your unit.
Double the width of your picture.


At this point, you have your unscaled picture in the clipboard and a scaled version in Preview.


Insert (choose Edit > Paste) the picture from the clipboard and align it to the left edge.
Open your second picture in Preview, select everything, and copy it.
Switch to the first picture, paste the second picture and align it to the right edge.
Save your new picture. Done.