Why two N-channel MOSET instead of one P-channel?
The two N-FETs permit the signal to pass through in a net non-inverted state. It also maintains the ability for the THERM_TRIP signal to be open drain to GND to activate - meaning that it can be easily wire-ORed if you have more than one source of over temperature detection.

A P-FET does not do these things in this same way.