Help me correct this circuit with N mosfet
Your reverse polarity protection approach seems correct (By the way, there's no need to put external parallel diodes for MOSFETs, because the MOSFETs do have diodes inside already). 

The configuration has nothing different than this:







simulate this circuit &ndash; Schematic created using CircuitLab

If you apply this to your circuit;







simulate this circuit

If the supplies are isolated then there will be no problems. But if supplies have a common connection at some point then some current (depending on the supply voltages and loads) may flow through the common ground connection between loads.