Pyrex said:
Why not? Except (possibly) in the case of an Intelligent weapon, I can't see anything in the RAW that prohibits a Holy+Unholy weapon.
A Holy weapon is good-aligned. An Unholy weapon is evil-aligned.
Damage reduction may be overcome by special materials, by magic weapons (any weapon with a +1 or higher enhancement bonus, not counting the enhancement from masterwork quality), certain types of weapons (such as slashing or bludgeoning), and weapons imbued with an alignment.
A creature with an alignment subtype (chaotic, evil, good, or lawful) can overcome this type of damage reduction with its natural weapons and weapons it wields as if the weapons or natural weapons had an alignment (or alignments) that match the subtype(s) of the creature.
So, DR X/good is overcome by weapons imbued with an alignment; that is, weapons that have an alignment that matches the [Good] subtype.
Either a Holy weapon cannot overcome DR X/Good, or it is a weapon that has an alignment; one that matches the [Good] subtype.
There are three alignments that match the [Good] subtype; Lawful Good, Neutral Good, and Chaotic Good.
DR X/evil is overcome by weapons imbued with an alignment; that is, weapons that have an alignment that matches the [Evil] subtype.
Either an Unholy weapon cannot overcome DR X/Evil, or it is a weapon that has an alignment; one that matches the [Evil] subtype.
There are three alignments that match the [Evil] subtype; Lawful Evil, Neutral Evil, and Chaotic Evil.
Assuming that Holy and Unholy weapons bypass DR, it means they have an alignment.
For a Holy weapon, that alignment is one of Lawful Good, Neutral Good, or Chaotic Good.
For an Unholy weapon, that alignment is one of Lawful Evil, Neutral Evil, or Chaotic Evil.
A Holy, Unholy weapon has an alignment; it is one of Lawful Good, Neutral Good, or Chaotic Good, and it is also one of Lawful Evil, Neutral Evil, or Chaotic Evil. Both conditions cannot be satisfied; the weapon is therefore impossible. They're incompatible abilities.
An Archon with an Unholy weapon could bypass both DR X/Good and DR X/Evil, because he is wielding a weapon that has an evil alignment, which bypasses DR
as if it had a good alignment. But the weapon does not have a good alignment.
-Hyp.