We rented a double a few weeks ago and it looks like the single just twice as wide.
We have twin 5 year old daughters and a 7 year old son. All three could ride in it. The twin sat 'normal' and my son sat sideways by the girls feet.
It was well worth the price as were able to stay from 8am until Midnight without the kids complaining about tired / sore feet. However, My wife complained a little and wanted to take her turn in the stroller...