How do I create a custom layout with two or more identically-sized photos on it?

You can select any photo (or empty frame) and copy it so that you have two or more identical frames on the page:

1. Select the photo or frame by clicking on it.
2. Click on the More button, then select Copy from the Edit menu listed there.
3. Click anywhere else on your page and select "Paste" from the same Edit menu.

You can then use the alignment guides to line up your photos.

Lastly, the new image well will contain the same image as the one that you copies, but you can then drag an image from the Media Tray on top of that image, and select Replace This Photo from the pop up menu to replace the image with the new one.