Tandem x 2!
I thought this would be a good thread in which to show this picture: My grandfather bought this 4-seater in about 1968. The picture shows my grandparents, and some of my siblings and cousins. That's me on the last seat.
A few years later when I was about 11-years-old, I would actually ride this beast by myself -- usually pretending it was a bus!
Motorizing this bicycle would probably require a Harley V-Twin!
None of us are sure whether it was custom built or factory made. And we don't know where the bike is today, unfortunately.