The Troll Slayer in my group has 2x Superior axes. The superior quality gives the bonus fortune die. But how does it work with double strike? Does he gain 2x fortune die or just 1. I ruled that he could only benefit from one.

It's up to you I believe.

I for one ruled that it was worth 2 fortune dice, why else would he spend the money on two axes when one superior and one mundane axe will give him the exact same bonuses? Fortune dice are a small bonus, so I'm quite free with them.

Obviously he only got 2 fortune dice when using "dual wield" action cards, not when using say reckless cleave or melee strike.

if he uses both weapons with an action i would rule that he gets both fortune dice, as mentioned above

we even house rule that you can also make a meele strike with 2 weapons, since you would not just use one weapon if you had 2 on your hands. damage calculation and damage dealt has to be discussed with this version though. we ruled that, with a "double meele strike" you would deal as much damage as the strongest 2-handed weapon in the game, i think that is 7, and with a sigmar's comet you would deal actual weapon damage. we calculate it like that: ST+weapon 1 dmg+weapon 2 dmg...

damage dealt would be somthing like dmg-TO-2x Soak...