Go read 50 or so light twin training accident reports and decide for your self.
Such as the report sqwkvfr posted where they are doing engine cuts below Vmc? Most NTSB reports result in people doing things improperly (almost always the result of bad judgment). Suggesting as you do that having a rear passenger (within weight/CG) is going to lead to a flat spin is disingenius.
One should not be scared of twin instruction provided one flies within the prescribed envelope. If you want to do full power on breaking stalls, cuts below Vmc, etc. Then having a rear passenger is the least of your issues. If you as an instructor are in fear of being in the aircraft then you shouldn't be instructing let alone in the aircraft.