For vehicles that do not lean during cornering, resistance to rolling over depends only on track width and the height of the center of mass. While the need to seat two abreast limits the width of most compact cars, it's the ratio of track width to CG height that limits the VW 1L and other tandems.
^^ That car needs a boat tail. Also note how they reduced ground clearance to lower the CG.
There are some unconventional approaches that might make for a better tandem car. You could have a faired, outrigger suspension like the Aptera's front, but detaching the wheels from the body raises your CdA, especially in a crosswind. Alternately, you could lean into corners like a bicycle or that new Nissan concept car.