When using the Oracle syntax below I need to update pairs of records
that meet some criterion:

Update
(
Select/*+bypass_ujvc */ t1.fieldA A, t2.fieldB B, t2.mark mark
from mytab t1, mytab t2
where t1.fieldC > t2.fieldC
and t1.columnD = t2.columnD
)
set A =B, mark=1; --<===

The phrase ,mark = 1 is not allowed because it updates the other
record of the pair.

IS there any way to update an Oracle table using pairs of records,
update both records simultaneously and preferably proceed in descending
order on fieldC?

This situation occurs repeatedly in a topological problem that
focuses on relationships between 2 things. Using another language one
would sort the records on columnD and then loop on pairs of records
with the same columnD value. If just records 1 and 2 have this property
then the problem is simple, but if there are more than 2 (say 3) then
we need to examine the relationship between 1 and 2, 1 and 3 and 2 and
3. If the records are sorted on columnD,fieldC we can simplify and just
process 1 and 2, then ignore record 3 if there are only 3 or process 3
and 4 when there are 4 with the same columnD value.

I have seen a use of a trigger to perform some "automatic" update of
the other record of a pair, but I don't know how to code that.
Thanks for any insights...