There is this really cute poncho http://www.lionbrand.com/patterns/kwec-mixAndMatchPoncho.html that I’ve been wanting to make. It’s basically just two long rectangles sewn together. It looks like it would be really fun, because you could use just one yarn to make the whole thing, or several. My question is this: why make two strips? It is all garter stitch, so maybe that middle seam would keep it from stretching too much?

It has 2 strips to create the shape. If you made one strip you would only have one point - which is still fine. It depends on what look you want. Sewing the two strips together gives it two points - sort of like a diamond with a hole in the middle for your head.

It seems to me the strips are just there to create two different sections of coloring with the yarn. I doubt the seam is important for any structural reason. You could just do one strip and change the yarns in the middle but the contrast probably wouldn’t be as striking.