Yes, since it is somewhat low voltage DC, diode ORing them would work. The diode would reduce the voltage sightly (maybe by 0.5V), but the detector can operate down to 8V, so all would be well. I would use a pair of 1N4007.
The downside is that this does not give priority to one source or the other (the one with higher voltage will be discharged). You maybe be able to use multiple diodes in series on the secondary supply to reduce its voltage to be less than the primary supply voltage (as long as the result is at least the required 8V). More complicated circuits with transistors could be constructed to better control the priority.
A few uF (10uF 50V) capacitor wouldn't be a bad idea at the output of the diode circuit. Ceramic capacitors would be preferred since electrolytic can have issues with temperature extremes.