[NeilBlanchard]

A search of "Llewellyn Sylph" turns up some music related results -- any other keywords that would help?

Jim, have you studied Ken Fry's Zing! which takes the tandem/enclosed fixed front wheel fenders path? Another option (depending on your local laws) is the Duo Quest by Allert Jacobs; which is a side by side vehicle with a pair of closely spaced rear wheels, and widely spaced front wheels. This may or may not qualify as a trike. An example of a standard trike with side-by-side seating is the TWIKE. I think the way it avoids the stability issues is keeping the speeds below highway speeds.

The main challenge of a trike is Cg. The location of the engine and the location of the people are the key things that determine the Cg. The geometry of the seating matter a lot to Cg. The height of the Cg matters a lot -- many tandem trikes lean in turns. Side by side trikes have very wide track on the pair of wheels, or longer wheelbase, or a closely spaced pair of wheels substituted for the single wheel (or a very wide single wheel).