combined. Well, on LA-area freeways where most of my experience with these is, it's either stop-and-go or 75+ in the carpool lane .
As you said you'd consider one over two cars - they are available in 12-and 15-seat configurations. The 12-seater is shorter, lighter, more nimble and gets better fuel economy, so if you can do with 12 seats reserve a 12-seater.