Photoshop?
Its not that difficult actually. Simply concentrate on what you want to achieve and google tutorials on that portion.
For this situation, the main concern I think is the aspect ratio. Keeping the aspect ratio correct means the photos will not be distorted and the subjects would look normal.
So, most likely you'll need to first decide if you are going to crop either of both or more photos. If you are, just create a file (in photoshop) with the paper being landscape/portrait 2x3 or 4x6. After which you simply fit both photos onto the entire white space.
If you don't want to crop either photos, you could overlay one photo onto the other (overlapping, changing the transparency of the top photo) or you could resize the photos and fit into the white space (leaving white spaces at certain parts)
This is assuming however that your photo is to be printed in the typical photo size (hence the photo sizes of 2x3 and 4x6). If it is just for memories, International paper size of A4, landscape, would be suitable.
Hope it helps!