Ah yes, you wouldn't be able to log any P2 then. Perhaps you can log the time to the FIR only, don't log any of the rest of the time, then in the remarks column put something to the effect of "P1 till FIR boundary."
Someone with better knowledge of your rules and regs will have to tell you whether you can have more than one P1 for a single flight though (i.e., is it legal to change P1 halfway through the flight.)
A question for you, are you REALLY P1 for that first part of the flight? If there is an in flight emergency, will you be handling it? If there is some weather, do you have the authority to call off the flight and turn back? Or does the other guy have ultimate responsibilty for the entire flight?
(I'm assuming "P1" is the same as "Pilot in Command", we don't use "P1" and "P2" in Australia.)