There were commercial kits for bolting two bikes side by side. I've seen one not on a bike but the owner wasn't willing to part with it at the time, plus that particular model looked rather flimsy.
The book Atomic Bicycle Builders Bonanza has a chapter on how to do the steering linkages for 2 front wheels. Keeping the front wheels parallel won't work. I see you have notes on just that issue.
When I built my first quad using 2 tandems, I considered doing them side by side but then I realized it would not fit in the garage door and would take up a lot of space. my Inline bicycle for 4 fits through the 32" entry door and the long and skinny quad doesn't hog as much space as a small car. There have been a few commercial side by side 4 wheelers. the Rhodes Car is one that comes to mind. Some with bench seats, some with 4 rider positions. I saw one on Ragbrai where they added an awning to keep the sun and rain away, a large cooler on the back, lights, sound system. It was a rolling party. They needed it all as their speed was very slow and it could take until after dark to finish each days distance. One big advantage is no balancing needed which can work for the elderly or handicapped riders.