I just started a campaign of Rogue Trader, and am new to it, so forgive me for lack of knowledge and names. Though I have GMed many other types of games, including DnD 3.5, Pathfinder, 4.0 and Star Wars. I was making a character that I thought would be interesting, and pretty much turn into a walking armory. And the creation went well, in my opinion. My main focus on combat was two Stub Automatic Pistols. I took ambidextrous and two weapon fighting (ballistics) to lessen the negatives, which if I recall correctly is -10 for offhand attack.

Now, our GM is a great guy, but a bit forgiving in some aspects. Another character built is arsenal very similar to mine. Even with two Stub Automatic Pistols. However, in game, we came to notice he did not take Two Weapon Fighting. Our GM apologized and said it was his fault, and gave it to him. Not Ambidextrous, and yet the guy was still onl taking a -10 for his offhand. We came to notice the same person only had 25 Agility, where you need 35 for Two Weapon Fighting, and the GM, again, let it slide. Its not that I want the guy to lose his Talent, or ability to dual wield, but it feels as if my character is being treated wrongly, as I actually took the traits, with the prereqs, and have even more, yet still taking the same negatives...

That isn't the main issue, however. I can live with those, despite not rightfully correct in my eyes. This game has attacks of half actions, where you can do another half action grouped in with the attack. Since an attack is a half action, the GM is stating you can attack twice as a full round action. I.E. a sniper rifle can attack twice in one round without any other actions (like moving or aiming). Same with a Pump-Action Shotgun, or a Power Sword. Note that no one has any other abilities like Swift Strike, Lightning Strike (I think thats what its called) or any other ability to attack more than once. He also states, even with Two Weapon Fighting, I can only attack with each one once per round. Equalling out to two attacks, just like anyone else is getting.

I looked through the books while it wasn't my turn, and couldn't find anything concrete to go against the ruling, but the rulings didn't seem to agree with his either. So I was hoping someone here could help me with my questions.

If this is true, I do not see why Two Weapon Fighting is even worth it at all. If I could have taken a Shotgun, which is slightly stronger, and fired just as often in a round, without dripping into so many talents.

Again, the GM is a great guy, I just want to know if this is the legit rules, because honestly, I am feeling a bit cheated for my character.

Seems like the problem is with your GM?

That isn't the main issue, however. I can live with those, despite not rightfully correct in my eyes. This game has attacks of half actions, where you can do another half action grouped in with the attack. Since an attack is a half action, the GM is stating you can attack twice as a full round action. I.E. a sniper rifle can attack twice in one round without any other actions (like moving or aiming). Same with a Pump-Action Shotgun, or a Power Sword. Note that no one has any other abilities like Swift Strike, Lightning Strike (I think thats what its called) or any other ability to attack more than once. He also states, even with Two Weapon Fighting, I can only attack with each one once per round. Equalling out to two attacks, just like anyone else is getting.

OK, from the top.

Rogue Trader core book:
Page 236 explains action. Notice under Half Actions, where it says that

A character cannot take the same Half Action twice in the same turn. Readying a weapon or making a Standard Attack are both examples of Half Actions.

Emphasis mine. So no, you explicitly cannot make two (or more) attack actions in a single turn, unlike eg. Shadowrun.

As for attacking only once with a pistol in each hand, you're explicitly not making a Standard Attack, but rather a multiple attack, p. 241, which references Two-Weapon Fighting, p. 246.

Go read that page. Then force your GM to read it.

But finally, you're using auto-pistols, in Rogue Trader!

My players have redshirts with plasma guns!

Tell your GM I said to Stop Worrying And Learn To Love The Bomb.

To be fair, the two-weapon fighting rules are a mess.

Regular joes with no special Talents take a -20 to any and all attacks with their "off hand" (ie the left-hand for right-handed people).

If you attempt to use 2 weapons at the same time, you take additional -20 modifiers to each hands attacks.

So that's -20 on your right hand, and a -40 to your left. That means that you will likely miss (unless you're shooting full auto at point-blank-range).

Two-Weapon Wielder cuts the penalty from -20 to -10, so now you're at -10/-30 instead of at -20/-40.

Ambidextrous loses the -20 for your off-hand, bringing you down to -10/-10 if you also have Two-Weapon Wielder.

At this point, you'll likely have spent some XPs on these Talents and their pre-requisites, so you deserve to kick ass

Its like in D&D. Without making full attack, you cannot make more than one attack per turn.
If your friends have a weapon with RoF S/2/-

they have following options:

1) Make a single shot as half action, and then have one move action left (which can be used before shooting to aim)

2) Make full action semi-auto burst, at +10 to hit, without moving. Or make full action semi-auto burst at +0 to hit, but can move up to his Ag bonus meters.

Also in DD when you met prereq for Feat, that feat didn't get any better, and penalty -2/-2 for TWW, still stood.

If its nicety due to noobiness in the gameline... Sure fine. But your GM is dead-wrong on most stuff, as has been explained above. Your buddy should not be dual wielding since his char is a bumbling fool when it comes to dexterity, the equivalent of having 8 Dex in DnD 3.5 and your GM should enforce this.

Personally in our group we use Black Crusade combat rules for all 40k RPGs, slightly better balance.

Swift Attack = Melee Semi-auto burst

Lightning Attack = Melee Full-auto burst

Semi auto and Swifts are done at +-0 and FAs and LAs are done at -10, standard attacks at +10...

Also all attacks in BC are made as Half Actions, aside from Called Shots, All Out Attacks and Charges who are Full Actions, you can also make a Swift Attack on the Charge - which is nice. Which leaves some room for maneuvering or something else...

Also, go get Gunslinger ASAP... And some Plasma Pistols, or Bolt Pistols, or Ripper Pistols... or some nasty Xeno-tech pistol weapon...

As for rules as written in RT... Making a Standard Attack is a Half Action, if dual-wielding you make one attack with main hand and one with off hand. Semi- and Full-Auto are Full Actions, if dual wielding you do a semi or full auto with both hands if the guns can do so. You can only do one attack per turn, unless dual wielding.

In melee you can only do Swift or Lightning Attacks with the main hand, while the off hand does a standard attack.