Sam wanted to take separate cars but he knew that that would tip Dean off. He still didn't realize that the other Sam had met Jennifer once before. Fortunately, Dean didn't seem to notice that either.
He let Dean drive the Impala since his brother wouldn't have it any other way really. And besides, Dean knew where they were going.
"So you and Jen?" he asked, with a raised eyebrow.
At least that was slightly more interesting than hearing about relationship drama.