For IFR practice, in the situation where both pilots are logging PIC,
one as sole manipulator, the other as acting PIC, can both pilots log
cross-country (assuming the flight meets the length requirements) for
the purpose of meeting the requirements for the Instrument Rating?

I'm not sure, but I suspect the answer may be "no", and one cannot dual
log all of the flight time either.

One can =be= PIC and still not be able to =log= PIC time. One may log
PIC time when one =is= PIC on a flight that requires more than one
pilot. IFR practice is such a flight, but only during the time the
pilot flying is under the hood, and (therefore) the safety pilot is a
required crewmember. Strictly speaking, this means that one (generally)
may not log PIC time when one is flying with a non-pilot friend and
letting that friend fly the airplane. Personally, I consider that
non-pilot friend to be an organic autopilot and I log the time. I'm
sure most other pilots do the same. It also means that in a safety
pilot situation, when the safety pilot is acting as PIC, but the flight
does not require two pilots (such as when the pilot flying takes the
hood off in VMC), the safety pilot who is also PIC may not =log= PIC
time. (remember, =being= and =logging= PIC are two different and
largely unrelated animals).

XC time useful towards ratings requires landings at two different
airports separated by specified distances. One cannot (in general) log
XC time for a flight that traverses thousands of miles and then lands at
the departure airport. This rasies the question of whether, in the case
of two pilots who each fly half of an otherwise legitimate XC flight,
either one could log XC time, if one does the takeoff and the other does
the landing. Similarly, it is not clear to me whether either pilot
could log XC time if one pilot does JUST takeoff and landing, while the
other pilot does all the cross country navigating while the first pilot
naps.

If the answer to this is "no" then I'd expect the answer to the original
question to also be "no" because (generally) one lands visually, and two
pilots are not required to land a spam can, so the safety pilot (acting
as PIC) could not be logging PIC time for the actual landing or takeoff.
It is true (and perhaps sufficient) that the safety pilot, acting as
PIC, might be able to log =flight time=, but I don't see any provision
in 61.51 for such time to be applied to ratings and such, since plain
vanilla flight time is not one of the kinds of time deliniated in the
section.

Now, what the regs =say=, what they =mean=, what they were =intended= to
mean, and how they are =followed= are four different things.

Jose
--
You can choose whom to befriend, but you cannot choose whom to love.
for Email, make the obvious change in the address.