Any Problem With Two PMOS Sources Tied Together?
If you want to make the turning-off voltage of the PMOSFETs "independent" of their rated \$V_{TH}\$, you can use a circuit similar to the following:



Transition is made after 1ms. You can reduce the forward voltage drop of the diode by using a schottky one. An output capacitor might be necessary if you want to overcome, or at least reduce the voltage drop which occurs during the transition.