problem with creating double photo slides in iPhoto slide show using vintage theme.

I'm creating a slideshow using the vintage theme in iPhoto. It seems to randomly group two photos into one slide. I like the effect and would like to create it myself. How do I do that? How do I take two photos, each in a separate slide and merge them unto one single slide?

Would love a tip. I've tried selecting each photo using the command key and tried "placing" them on top of each other. Sometimes I get a red "2", but they still don't merge unto one slide - they stay as two separate slides.

Help please.