Here's what the FAA Chief Legal Counsel said about it more than 10 years ago in 1992:
In your second question you ask "how shall two Private Pilots log their flight time when one pilot is under the hood for simulated instrument time and the other pilot acts as safety pilot?" The answer is the pilot who is under the hood may log PIC time for that flight time in which he is the sole manipulator of the controls of the aircraft, provided he is rated for that aircraft. The appropriately rated safety pilot may concurrently log as second in command (SIC) that time during which he is acting as safety pilot.
The two pilots may, however, agree prior to initiating the flight that the safety pilot will be the PIC responsible for the operation and safety of the aircraft during the flight. If this is done, then the safety pilot may log all the flight time as PIC time in accordance with FAR 1.1 and the pilot under the hood may log, concurrently, all of the flight time during which he is the sole manipulator of the controls as PIC time in accordance with FAR 61.51(c)(2)(i). Enclosed please find a prior FAA interpretation concerning the logging of flight time under simulated instrument flight conditions. We hope that this interpretation will be of further assistance to you.