Dualwield vs Twohanded
 

Hi i was just wondering, for the barb class what is better Duelwield or Twohanded

Generally dual wield.. it's not very often you see barbs with a two handed weapon these days. Dual wield is faster and provides very good damage, so you're looking at more DPS compared to two-handed weapons. Also, because you technically have two weapons, it also means an extra slot for a gem. But it's all up to you. If you find a two-handed weapon you like, then by all means go crazy.

Should i use 2 Swords or 2 axes 2 hammers? what do yo recommend

Depends on which you already have that is best weapon, & also, your choice of attack spells. I'm a ms spammer, many addicts love sc, etc. A really big slow or very slow hitter can max out your fulm/exp wave at doors, but few skill that combo anymore...

...& yes, wielding 2 weapons is generally better than only one. NGD wants to sell you boxes (lots of 'em), & getting 2 prime weaps is more than twice as hard as getting just one. This is THE reason 2-weapon wielding is better-by-design than 2-handed weaponry.

Never tested, so not sure if it's true, but several expert players have told me that the only things that matter in an offhand weapon are it's damage, it's specific bonuses, & it's weapon speed. The weapon (damage-type) itself is irrelevant. Is that true, peanut gallery?

Totally up to you. Axes (Elite Knight Axe in particular) and spears are the most popular. Spears for their decent damage and longer range, and the axe for south cross. This spell does high damage with a low cool down.
The off hand doesn't matter so much, because only your main weapon is taken into consideration in regards to the spells you can use. For example, if I'm wielding a Spear as main and mace (blunt) as my off hand, I wouldn't be able to cast Mind Squasher. So just find an offhand with good damage.
I wouldn't use two swords, just because swords don't do as much damage. Axe/sword, Spear/Mace combo are pretty nice, but it all depends on the weapons you have access to and what skills you want.

Off hand weapon brings more ballance to classes by adding extra bonuses to beat shields and arrows. Don't ask about mages as long as they have extra spell tree.

Extra spell tree? Be more precise, staff mastery or enchatments? Can't decide which one is more usable.

Neither. Mages get 3 additional spell trees after choosing subclass. Other classes - just 2.
It doesn't matter if you do not use them. You still have extra power points.
This difference between classes makes balancing harder.

you cant put a gem into shields (ofcourse) or arrows, though. now we have an imbalance again :P

PP sinks for basic functionality, to cover much higher cast times, higher mana cost and lower mana regeneration and not to mention basic damage dealing.