Ehhhh I don't know about that. Humans are fragile, but in my time, in my dads time, and in my grandpa's time, we've never heard of a shoulder harness tearing someone in half in a crash. It can forsure lacerate you if the impact is hard enough, but not cut you in half.
Shoulder harnesses don't have the best mounting mechanisms either.
http://ftp.rta.nato.int/public//PubFullText/RTO/EN/RTO-EN-HFM-113///EN-HFM-113-03.pdf
It even talks about fatal injuries; with and without single shoulder harnesses in there, and i think that 76% of the time they had found shoulder harnesses to fail.
The alternative to not wearing a chest belt, single or double is slamming your face into the control yoke.
Cessna has those covers on the centre and they are soft and squishy to take up impact forces, but the fact is is that the handles on the yoke simply aren't wide enough, so you are pretty much guaranteed to smash your face down onto that, and I've met a a dude who was done lucky enough to survive. He had steel plates in his head because of multiple skull fractures and a glass eye. Not cool at all.