Activating the Seeking Sword and equiping a secondary weapon allows the Priest of Helm to receive the benefits of Master Two Weapon Style and Single Weapon Style simultaneously. This should not happen. It's expected that the Priest can only ever benefit from one proficiency in Two Weapon Style, and the Single Weapon benefit should never be active while the Priest is dual-wielding.
This problem occurred regardless of which secondary weapon I used, but the class functioned normally if I equipped a shield instead (i.e. I got the Sword and Shield benefit only).