different sort order for primary key index

I have a composite primary key for a table, let's call it (col1, col2).  When this table is created, obviously an implicit index is created for this key.  I would like the sort order of this index to be different for the two columns -- if I were to create the index myself, I would pass on (col1, col2 DESC).  The ALTER INDEX  documentation suggests that it's not possible to change the sort order of a column, so I can envision two ways to get around this:  1) create a second UNIQUE index of (col1, col2 DESC), or 2) not define a primary key and just specify a UNIQUE index separately.  Primary keys are basically restricted to being unique and non-null, but I'm unclear if PostgreSQL treats primary keys differently from unique, non-null constraints.