Need to keep weight inside triangle to prevent roll over
Good point. However, any side-by-side passenger arrangement makes this pretty much impossible - unless the people are sitting directly on toP of the front wheels (like how the Trimuter is designed). You have to take into account whatever front suspension you use, as this takes up space.
In this particular design, I'm using a stock VW Beetle front end axle. So, with a side-by-side system, you simply cannot move the passengers any more forward without their feet interfering with the wheel wells. Even a short double A-arm suspension is going to take a good amount of space - unless you want a significantly wider wheel-base. This is certainly doable. However, I'm trying to design something using as much stock mechanics as possible. Designing a suspension system can be a bitch to engineer correctly.
On the other hand, if you go with a tandem setup then you end up lengthening the vehicle (or raising the position of the passenger as in the post above, effectively raising the COG) - and you end up with similar instability problems. Unless of course you go with a tilter - which adds significantly to the complexity.
So it's a catch-22 situation.