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If you chose to two-weapon fight (that can only be done as part of a full-round attack action), your off-hand is busy for that entire full-round giving you the benefit of TWF (the single extra attack). It (your off-hand) is unavailable to assist your primary hand in making a two-handed weapon attack.
In other words, although your primary hand and off-hand attacks are resolved sequentially, they are occurring concurrently. Your off-hand cannot be involved in the two-handed attack AND the off-hand attack during the same round if you are TWFing.
The feat Two Weapon Fighting simply reduces the penalties for using TWF.
The feat Improved TWF and Greater TWF simply increase the number of offhand attacks one gets when using TWF (albeit at an increasing penalty).
They do not change the fact that your off-hand becomes dedicated to TWFing when, at the start of your full-attack action, you declare that you're going to TWF.