As a GM I have a question: when firing two weapons in full auto with two weapon wielder talent, the rulebook and errata states that a character recieves a -20 penalty to both hands. Now here is my question - should I treat additional range bonuses (as penalties are rather obvious), for example range or weapon modifications, for this action, or should this attack always be made with a -20 penalty with any additional penalties for range, close combat, fatigue, wounds etc? What are your opinions on this matter?

Yes, I apply the modifiers. One thing to point out though, what weapon modifications are you using to get a bonus on full auto? The CRB as I remember has none, and most of the bonus to full auto attacks I'm reasonably certain have a requirement of being a basic weapon. Which, while they can be one handed, it has been found to be quite cheesy to dual wield basic weapons.

Well if the basic weapons aren't pistols then they recieve a -20 modifer for using them single handed. I'm not sure if the range finders and laser sights should be taken ainto account here. Full auto dosen't really rely on them.

Normally you can not use weapon scopes on dual wield, but you can use laser sight, however laser sight only applies to single shot mode.

Apart from that you add up all the modifiers to max +60/min -60 as per errata. The errata is rather long and has a lot of updates on combat - especially dual wield, so you should go through it in detail.