The easiest way would be to batch process all your photos and put them in two columns.

OR

The other way would be to resize your images and put them on the same canvas, creating one image. Then make thumbnails and put them in a Lighbox or Fancybox gallery.

Links (I have tested the top two on previous client work):
http://fancybox.net/
http://leandrovieira.com/projects/jquery/lightbox/


Haven't tested this one, below.

http://www.huddletogether.com/projects/lightbox2/