Each time you gain a level, you gain the action points described in the class.
These stack with the action points you still have from previous classes (note, that only applies to D20 Modern. In the Eberron Campaign Setting, the action points are reset each level).
So, yes, the action points of different classes stack. A Fast Hero 1 / Strong Hero 1 would have 11 action points, provided he never spent any of them. But so would a Fast Hero 2 or a Strong Hero 2.
Once spent, an action point is gone, which means that most characters that are exposed to the usual harm of the adventuring career (facing critical saving throws or important attack rolls and skill checks) will never pile up that many. (If characters do, it probably meant that either the challenges they faced weren't very hard, so they didn't need them, or became harder, because they didn't use their action points...)
