One long cushion will often have the problem of the fabric "rolling" on the cushion so the seams don't stay at the front of the cushion.

On a two cushion sofa, the "middle" person will be sitting on the crevice between cushions. However, people rarely voluntarily sit in the middle of a sofa between two other people. I tend to like two cushions. I don't like the way that most current 3-cushion sofas are divided, which makes the center cushion look too skinny compared to the end cushions.

We prefer two.
They are easier to handle than one large cushion.

As pal mentioned, people rarely will sit between two others, so having that seam in the middle is not much of an issue.

I prefer two cushions as well