FAR Part 61.57 (c) Instrument experience. Except as provided in paragraph (e) of this section, no person may act as pilot in command under IFR or in weather conditions less than the minimums prescribed for VFR, unless within the preceding 6 calendar months, that person has:
(1) For the purpose of obtaining instrument experience in an aircraft (other than a glider), performed and logged under actual or simulated instrument conditions, either in flight in the appropriate category of aircraft for the instrument privileges sought or in a flight simulator or flight training device that is representative of the aircraft category for the instrument privileges sought—
(i) At least six instrument approaches; (ii) Holding procedures; and (iii) Intercepting and tracking courses through the use of navigation systems.
So, for the safety pilot to be PIC (so both can log) that person must be IR and current (in order to fly IFR - to do practice approaches etc.)
The letter you reference is from 1992. The regulations were changed in 2009 or 2010.
If any portion of the flight were conducted on an IFR flight plan (e.g., in and out of the clouds and/or even on an IFR flight plan) at least one of the pilots must have an instrument rating and the §1.1 PIC must be instrument current in accordance with §61.57(c) and be Flight Review current in accordance with §61.56(c).