Im curious if their are rules are for taking actions for which a player doesn't have actions for. For example lets say I have a player that has dual pistols, but doesn't have the action card that allows him to shoot both simultanously but calls it anyway "Hey I want to shoot both pistols at the same time in the same round".

Im hesitant to say "cant do it unless you have the action card", but in the same token if I allow it.. whats the difficulty? How much damage should it do? … I mean its technically a ranged attack but I think the presumption of the player is that if I shoot both their should be advantage if I make a good role (more damage etc..).

This is just an example but Imagine my players will be doing a lot of "self defining" actions for which there are no action cards or they simply dont have an action card for. Are there any official rules on how to handle these situations?

official rule is use the "perform a stunt" action card and let the GM set the check difficulty.

i would say, if the player has 2 pistols and wants to use both, why not with a sigmar's comet. that way he can try if he would hit with both and if he rolls the comet he does, hitting for +secondary weapon's damage for example

i rule sorta like that for a twin weapon character, if he rolls a comet on a meele attack he will also hit with his secondary weapon or, depending on the rest of the result may perform another meele strike, enabling him to make even more damage and letting him really "attack with both weapons".

or you can just increase the difficulty of such a check, by 1. so a check with difficulty 1d has a mod of 1d itself and for using 2 pistols you will add another one, that makes it a 3d check. its hard, but doable!

I would use the Perform a Stunt card most likely. Generally if you know there is an action card out there that has the attack on it, you could use that card still but increase the difficult. Maybe convert black dice to purples, or increase the number of difficulty dice. At the same time, you could increase the effect of any chaos stars rolled as well. Make them a bit worse just to show the players that you can try attacks that they are not familiar with, but the penalties are steep and the price of failure is higher.

i use perform a stunt unless i happen to know and can quickly find an existing action card. if there is an existing action card i let them use it but i add extra purple dice to the attempt and do not allow them to use the higher success results.