Parallel Power Supplies with Ideal Diodes
You need two back-to-back P-channel MOSFETs for each power input. Here is an example from an Atmel eval board, though you'll want to scale up the MOSFETs considerably, and maybe reduce the resistor values to something like 10K: 



The VCC_USB input is dominant if both are present (in other words, if only one supply is present, that one will be used, if both are present, then VCC_USB is used). 

The back-to-back MOSFETs are "ON" when the common gate connection is at ground (assuming the left source is high) and are off with the gate connection equal in voltage to the source. 

Because of the intrinsic body diodes you need two MOSFETs to be able to block in either direction. This is the common configuration you'll see in hot-swap controllers and similar devices. 

The connection to R613 turns Q109A/B on if VCC_USB_EDBG is low. If both supplies are present Q110A/B are off (because their gates are high) and Q109A/B are on.