Well, you can, and you cannot. There are a couple of ways to do it. It's easiest under the FAA.
1) If one of you is an instructor, and the other one is undergoing training for a certificate or liscence, and the student is already qualified to fly the airplane, and the student is "performing the duties of PIC" both may log PIC. Ie, one of you is a PPL holder, and one of you is a CFII, one will give the other one "instrument training".
2) Operating an airplane "under the hood", as may be required for instrument proficiency, requires a rated safety pilot. So say that person A is a PPL IR, nominated to be PIC, person B is a PPL (with or without IR), and acts as a saftey pilot. As a required flight crewmember, this person may now log second in command. (Note: the PIC must be the one under the hood -- if the PIC is the safety pilot, the person under the hood is no longer a required crew member).
3) In JAA-land, at least, a regulator may authorise MPO in a SPA -- a multi-pilot operation in a single-pilot aeroplane. I.e. the Beech 200 "King Air" is a single-pilot aeroplane, however, in my company, we must operate it multi-crew, and the copilot, will log the flight time as such. (The logic being that the copilot is a required flight crew member, required by the regulator, even if not required by the manufacturer).
Last edited by bfisk; 26th Jan 2009 at 15:16.