The reports I have read about stacked batteries seemed to stress the importance of equally charged batteries and checking them with a voltmeter each time off the charger before installing them into a mod. If you screw up and install one depleted battery along with a fully charged battery, then press the fire button, the higher charged battery will rapid discharge into the depleted battery. Heat and pressure is the result and Li-ion batts can vent and spew. The electronics won't detect the things programmed into the logic, like shorts and voltage out of range and the meltdown will continue.
I'm not sure that a sacrificial spring would fail unless a dead short was experienced. That's what the spring is for. Anyone using dual batteries needs to check them with a voltmeter each time to make sure that they are equally charged.