I don't know why, but you just can't. Sorry, this isn't a great answer, but the photo capsules just aren't made to work side by side.
You could rig something up using Photoshop or MS Word if it were really important to have two pictures side by side, but it is time consuming. I did this for one of my hubs (Make Something for Nothing - Crochet Fun Beach Bags...) using MS Word because the step-by-step pictures were more effective seen together, but it was rather a pain to do. The end result was good, though.