Circuit with dual power source: Replacing diodes with MOSFETs
I've little experience doing this in practice, but in two designs I'm working on, I am and you are looking for something like this application note for similar reasons: https://www.maximintegrated.com/en/app-notes/index.mvp/id/636 

You should be using enhancement devices, with PMOS gates tied to ground on the high side -or- NMOS gates tied to Vdd on the low side.