On Casual and Normal, double archer can be quite devastating for most areas. Most enemies will die before they get too close. Some boss fights can be a bit brutal as those enemies can close pretty fast (speed potions can be a lifesaver).
On Hard and Ironman, the double archer plan is hindered a bit by the increased defense and attack of enemies.
I think the stun effect from Archery Skill is under-appreciated on this form. Most players seem to go the route of Assassinate. Two archers can stun-lock tough enemies in a surprising amount of times.