Another real-world example: I'm a type-rated FO at a fractional. I log my legs as PF as PIC as the FAR's allow. The company backs this up by stating during training that if I upgrade on the same aircraft and I have logged 100 hours PIC in the aircraft I won't be a high-mins captain, even though I have never signed for the plane.
You can't use the PIC logged as a SIC to waive the high mins for captain under part 121 (not sure about fractionals). You were not serving as PIC...only logging PIC.
The second way to log PIC flight time that is pertinent to your
question is to be the sole manipulator of the controls of an
aircraft for which the pilot is rated, as you mention in your
letter. Thus, under a 121 operation you can have both pilots
logging time as pilot in command when the appropriately rated
second in command is manipulating the controls.
We stress, however, that here we are discussing logging of flight
time for purposes of FAR 61.51, where you are keeping a record to
show recent flight experience or to show that you meet the
requirements for a higher rating. Your question does not say if
the second pilot in your example is fully qualified as a PIC, or
only as an SIC. This is important, because even though an SIC
can log PIC time, that pilot may not be qualified to serve as PIC
under Part 121.
An example of this difference is FAR 121.652(a), which raises IFR
landing minimums for pilots in command of airplanes flown under
Part 121 who have not served at least 100 hours as PIC in that
type of airplane. Served and logged are not the same in this
context, and no matter how the SIC logs his time, he has not
served as a PIC until he has completed the training and check
rides necessary for certification as a Part 121 PIC.