On our B737-400 BOTH engine & APU detectors have their respective fault lights.However only the APU DET INOP light triggers the Master Caution annunciator.
On the Q of to go or not go with both detectors failed, we had an incident a couple years ago on the 747-400 where both detectors on an engine had failed in flight.The commander elected to return to base, the company however wasn't too pleased with his decision & convened an inquiry.
To keep it short, the question of appropriate response to this problem was directed to Boeing in the course of the inquiry.Boeing's response after some technical gobbledygook was "at commander's discretion".
Cheers!
[ 06 October 2001: Message edited by: steelbird ]