Switchover between two power supplies using FETs
In the circuit you show you will not be able to control M1 or M2 since you have not correctly interfaced the gate voltage relative to your input voltages.

I think what you might be able to copy is the schematic of the Si4700DY (now obsolete but they give a good schematic) http://www.vishay.com/docs/71110/71110.pdf

In this schematic they ideally need VG pulled up to a value equal or greater than either source voltage. In practice you should be able to use two low VF diodes (such as the SB320) and still ensure the FET can be turned off. 

You'd end up with something like this: