What's an iPhone app that has it so you can put 2 pictures together in 1 picture?

Like say you add a tree to the left side. Than there's a line that separates the one on the right side which could be a water fall. Gaaa I can't explain it well hopefully you know what I'm talking about haha

Pic Stich is one, or Pic collage.

Diptic is a really good one.

Instacollage is awesome! It has different borders and designs.

It is called as collage where multiple photos are arranged in different manner to create a single photo.
I would recommend Diptic, Photo collage, Pic/Photo Collage, Cool Photo collage, Pic Jointer etc…

Hope this helps.