How does the two weapon fighting feat work when (for example) a fighter reaches 6th level and gains a bonus attack roll per round. Does this mean the fighter would get four attack rolls per round? Two from his two wielded weapons and two extra attacks with each. Or is the bonus separated to just one more attack?
You don't get extra off hand attacks.
You are skilled at fighting with two weapons.
Prerequisites: Dex 17, Two-Weapon Fighting, base attack bonus +6.
Benefit: In addition to the standard single extra attack you get with an off-hand weapon, you get a second attack with it, albeit at a –5 penalty.
Normal: Without this feat, you can only get a single extra attack with an off-hand weapon.
Get this feat and it's greater counterpart as you level up to gain extra iterative attacks with off hand.
You can hold a weapon in your off-hand (or use armor spikes or the like) and gain one extra attack per round using a full attack action. This applies from level 1 to 20. BAB only increases number of attacks for your main attack.
Two Weapon Fighting feat merely reduces the penalty for this attack, but does nothing to increase total attacks per round.
Improved Two Weapon Fighting adds a second iterative off-hand attack.
Greater Two Weapon Fighting adds a third iterative off-hand attack.