We don't but are there many scenarios are there that this could actually come into play? If both flight crew are completely incapacitated due to impact injuries, then that would suggest fairly heavy damage to the flight deck and possible buckling of the front section. Would the crew even be able to get in and reach the handles? That's assuming that the cabin crew at the front are also not incapacitated, the engines have survived the impact and the controls are working. The only other situation I could think of would be smoke inhalation but again, if the flight crew are overcome and those in the cabin aren't, would the time taken to get into the flight deck would be too long?
(These are genuine questions by the way. It's a subject I have an interest in, not just for my own personal safety!)