I’ve ridden both, but not in many years. Loved it as a kid, not as much now. I got a Space Mountain T-shirt on my first trip to WDW in 1976.
I remember those seats on the original MK ride. It was a bench seat that you straddled, and like you say, fit two people in each section, with just the loose seat belts, and hand rails on the sides. The person in front essentially sat in the lap of the person behind. For a single rider, especially a child, there was lots of extra room in the compartment to rattle around. My only memory of the ride at
Disneyland is that it has the different side-by-side seats, so not as rough.