When using a spear and another weapon, can the player use surges to activate the second weapons ability when 1 space away from monster?

For example if a hero are 1 space away from a monster using a spear (for reach) and a light hammer in his other hand (for stun). Can he use 1 surge to inflict stun on that monster?

No. You have to choose your weapon before attacking.

You have to delcare which weapon you are using before rolling dice, and the rules say you can only use surge effects on the weapon you chose to use for this attack.