Motorcycle sidecars were invented for people who couldn’t afford a car, not because they were dynamically graceful or elegant in design.
Given the amount of tinkering that bike enthusiasts do (trikes, truck engines, trailers, etc ) I’m slightly surprised that I can’t find anyone who has developed today’s invention.
It’s simply to attach two motorcycles together in parallel, linking their steering, gearchange and throttle mechanisms so as to create a dual-control machine. The linkage would be a parallelogram in section, allowing the bikes to lean over to the same extent in bends.
Why? Well, first you get lots of the benefits of a small car (eg stability, especially if there are frost patches) but most importantly it provides a way for one experienced rider to teach a less experienced one how to take corners etc safely at speed (something which most riders either never learn -or crash while attempting to discover).
This would require two identical machines to start and there’s not much point in using ‘boxers’, since the cylinder heads are going to be in perpetual conflict. It would, however, be useful to motorcycle training schools, or even as promotional vehicles -Tom Cruise could appear to do more of his own pointless stunts, without having to stick to a straight line.