Spitfire,
It all depends on where you are time building and for what ratings?
Under the FAA regulations, all the flight time post PPL, even with an instructor, is P1. If you are flying under the hood, practicing instrument flying, and there is another safety pilot, both of you can log the time. Provided safety pilot signs your logbook and you sign his logbook. All the dual time towards your FAA IR is P1 under FAA rules, the only time you cannot log P1 is when you are in actual IMC conditions.
Under JAA/CAA rule, you cannot have two PIC's for a single pilot operation. All the dual time towards IR or IMC is still Pu/t.
So if you are in USA working towards FAA ratings you can log the time as P1 (ofcourse one of you under the hood) but these P1 hours cannot be counted towards your JAA CPL-IR.
Jatin