Redundant DC-DC converter (parallel)
Unless this is a regulatory requirement, you probably ought to make the converter more robust rather than doubling up. If one dies, the other is very likely to die of the same cause.

Rather than adding shut off FETs, just use a higher voltage rated part e.g. TPS5420.

With 3s you'll have~12V, which means you'll only have 5V headroom. RC stuff can be rather noisey, and switch mode converters are like electronic hammers. Your Vin should have a ceramic decoupling capacitor as well as the 100uF electrolytic, 10uF ish and I'd add a ferrite bead to damp the high frequency ringing, spiking and emissions.

Honestly, I think you can attain much better reliability with half the components...