APU does only have one loop, and a fault produces the light to illuminate and Master Caution etc. This doesn't happen with the engine overheat/fire detection loops, whether one fails and you have selected the corresponding loop or whether both have failed and you are in Norm. Only indication of a failure is a FAULT light. As Checkboard has mentioned you are not going to shut down a functioning engine in flight due to no overheat/fire detectors on one engine, you can still monitor the engine parameters and you would suspect you would be told PDQ if flames or smoke was seen in an engine.
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