If you fed two plugs off a single coil only one plug would fire most of the time
That's not strictly true. As long as there is a circuit from one terminal of the coil back to the other terminal, it'll fire. The second terminal doesn't have to be a chassis/ground potential.
I can name at least one case where the circuit goes coil+ve -> spark plug1 tip - >spark plug1 threads -> cylinder head ->spark plug2 threads -> spark plug2 tip -> coil-ve.
Both spark plugs 1&2 fire every time at the same time. One cylinder is at the top of the exhaust stroke, whilst the other is at the top of the compression stroke. Although the exhaust stroke spark is wasted (hence the term wasted spark), it takes very little energy to fire it when it's surrounded by hot combustion gasses, so the energy loss is minimal.
It makes spark timing simpler - you can do the timing off the crank (rather than the cam) since you don't need to know if the piston is in the compression or exhaust phase of the 4 strokes.
No idea if that's how a Smart car works though.