Also, two of the same weapon stack in the sense that you get to attack with both of them every turn. (If the weapon has other effects like resistance or magic boosting, I think that is not stacked, though. Possible exception for 3+ arms and a similar situation to what Edi describes for miscs.)
If you want more fear than the horror helmet alone, there are several other item options for fear. Implementor axe, skull standard, standard of the damned, both axes of rulership together, lantern shield, aegis, and the black book of secrets all give some fear (some of those are artifacts though - without artifacts, I don't think a humanoid can get more than +5 just from items). The Gift of Kurgi gives a *LOT* of fear, but has some pretty major drawbacks.
