Not shortened or lowered. It was two different bikes welded together.
Based on the decals, the front of the frame looks to have been a Schwinn Woodlands mountain bike made of Reynolds 500. 700c road fork, road wheels. Back half? Don't know. It's on eBay. You could email the seller to see if he has any photos with riders on it.
In the original post, @Rickpaulous
mentions that he built a quad some years ago. Last year I came across a couple of Murray middleweights that were really cheap so I bought them for the accessories - tires, wheels, handlebars, etc. I drew up some plans to make a quadracycle from them. Has anyone done this? If so, what are/were the pros and cons of the build. Here's a picture of my drawing.
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.