I have Two Weapon Wielder and Dual shot talents, and no penalty reducing talent as Ambidexterous or Gunslinger. When I use Full round action to use Dual Shot, i make single attack and if i hit, i do damage as both shots but reduce the Toughness bonus once. So far so good. I need some clarification here.

The question is: do I get -20 penalty on the roll?

My rule at the moment is that player, that use this talent in combat gets -20 penalty. Is that correct?

No. Duel shot gives no penalty. Not having a penalty is offset by the fact that the entire success of the attack is dependent on one attack role and the fact that both shots can be dodged with one dodge role unlike two weapon wielder. This is covered in the Errata.