How to combine and protect two 12V automotive signals into one?
Q2 is effectively a diode already so no additional diode is required.

I'm not too convinced about the clamping circuit though and I don't see a pull down on that enable line so it's effectively floating when off which is a no-no according to the data sheet.

I think I'd build it something like this instead. D1 ensures the enable can't wander above the power rail and R1 limits the current through that wire if it does. R2 pulls the enable low when everything else is off. 

Not sure why you need C1 though. It will cause a significant delay on disable, especially at 10uF.







simulate this circuit &ndash; Schematic created using CircuitLab

ADDITION: For reverse battery protection too, this is probably better.







simulate this circuit