A bastard sword can be wielded with two hands or one hand only right? So if a fighter wield a bastard sword and have 2 slots in one weapon hand and two-handed weapon skills which of the skills will be considered? Both of them or only 1 of them?

this has no affect in any basic version of bg2:SoA orToB. a bastard sword is always a 1handed one. to fix this you have to install a component of weimer's ease-of-use mod (i think). both style skills should work then.

You can download ShadowDaemon's 1-Hand/2-Hand Bastard Sword mod here http://www.sorcerers.net/Games/BG2/index_editors3.htm