We do offer a handful of pre-made MOO designs that allow for two or more images to be added onto to one card - you can filter your search using the “use my photo” toggle on the left hand-side of the browse designs page. The most popular of these designs is called Contact Sheet.
If you are using MOO's tools to "Design your own" only one image is permitted on the details side. The reverse does have some multi-image layout options, which you can scroll through via the layout menu at the bottom of the design tool.
You can also use Printfinity to add multiple images to the back of your cards.