Power ORing using P-Channel MOSFETs
PMOS-based power OR-ing solutions exist, but the biggest problem is shoot-through when the power supplies are at different voltages and both PMOS transistors are active. MOSFET transistors conduct in both directions when active. A correct solution requires that both supply voltages are sensed, and the PMOS transistors actively switched on and off to prevent shoot-through.

Here is a simplified simulation of your proposed circuit, showing that most current is flowing from the 15V supply into the 12V supply. This could cause damage or even an explosion if one of the supplies is a battery.