Age 7 and up can ride entirely alone, so yes they can ride together....or apart.
During our last trip, my Dad, my son, and I rode Test Track a few times in a row via the single rider line. We put him in the middle of us in the line, so one of us would be waiting for him when he got out. It actually always worked out, so that he was IN the car with one of us, but he wasn't in the same row.