I would use whatever adhesive you normally use to attach paper to paper. For me that would be double stick tape or more specifically-a tape runner. I would probably go all around the edges to make sure the two pieces stayed together well and I didn't get one corner or edge that popped up.
How about stitching them together for a different effect. I'd generally use a narrow double side tape on the edges but you have to make sure the edges are straight and line up. if when you scrap the Lo you use a few brads this will help hold it all together. Or more sticthing.: )