Powdermonkey, while not sure about the JAA regs, under FAA regs you certainly can. PIC (or P1 as you call it) is defined as 'sole manipulator of the controls of the aircraft'. The PIC role can pass from one to the other as many times as you like during a flight and where you sit is irrelevant (well, as long as it's somewhere from where you can reach the controls
).
Also, t/o and landing don't come into play here. However, you must have the approriate rating, i.e. be legal to fly the a/c in question (rating, medical).