We are sure you have seen this before somewhere recently but if you give your Slaanesh Herald (on foot) the Torment Blade, does this mean he can fight with 5 attacks instead of 4 due to now having 2 handweapons(Warhammer Online Gold)?
The Blade is a hand weapon, bought in addition to the hand weapon every model starts the Warhammer with. The rules of Fantasy dont make distinction between which of the 2 weapons hit. So all 5 attacks use the ability of the 'magic weapon' (Gift in this case).
Because a daemonic weapon isnt like a magic weapon which said by somewhat misleading. It doesnt give your character(Warhammer powerleveling) a bonus in combat, it improves your daemon by adding stats or rules.
Most of the time when a magic weapon is combined with a mundane one (this is reserved for special characters, mostly), the extra weapon only adds one attack (with the bonus of the other weapon applying to it).
When adding extra attacks from other sources (maiming shield, many limbed field), these attacks are resolved at the base strength of the wearer, with no extra rules from magic weapons. The exception to this is the kickin' boots in the O and G list.