Disciplines are the 'schools of magic' for martial characters. They have maneuvres, which are like spells.

ACF stands for Alternate Class Feature, in other words, different abilities in place of the standard class features.

Too strong? I wouldn't think so. It would still require time to switch personalities, even more so if one of them is a spellcaster (still needing an hour at least to meditate/study/whatever, maybe even requiring the full eight hours of rest). In the same way, a fighter personality would have to put on the armor, weapons etc.

Also, if you were to use different classes, you wouldn't be able to go gestalt with them (a.k.a. combine the abilities of two classes, keeping the best), because you will still have only one class active at a time, and damage would get transferred over.

In the end, all you get is two different characters in the body of one, but keep in mind that can be a bad thing; you would not get two 'sets' of stats, just one which will have to work for both. In my opinion? With a decent DM and party, this can help keep the interest for a bit more time, after all, even playing a wizard gets boring after too long and this way you can switch to a fighter to get some fresh air so to speak.