Isn't that why they are tested at the start of each flight duty? And if they don't work, maintenance is going to fix that or the plane is AOG. Not to mention, the doors unlock in a decompression.
Anyway, all german airlines will stop that procedure, latest in june. The EASA recommendation requires a safety and security assessment and, if it is positive for the 2 person rule, the introduction of that procedure. That assessment was made after the 2 person rule was introduced and, not surprisingly, showed that actually the new rule was quite a bit less safe than the former procedure. However, german airlines were of the opinion that public pressure was so high that they had to use the less safe option anyway. Now they justify the abandonment of the two person rule with the introduction of random alcohol, drug and medicine testing (any medicine, no matter if over the counter or not, taken without prior consent of the AME is illegal and can lead to immediate dismissal). And of course with the fact that suicides are a lot less likely than hijackings.
As the german LBA required CCTV with at least three different camera-views of the flightdeck entry area for a reinforced door, as well as the possibility to open to door without leaving a seat, there is no practical reason for a second person to be inside the flightdeck if one pilot leaves. In a decompression the door will unlock automatically, in the case of an incapacitation of the remaining pilot the emergency code will unlock the door as well.