While @Jadasc's answer is correct (by RAW), my own personal ruling is slightly different.
For sake of keeping things clear and simple, my house rule is:
Unless specifically allowed by the rules (Such as a combined fighting style), in a single round of combat, manoeuvres/bonuses from
different fighting styles can not be used together.
Not only do they not stack, you can't use them together at all. Even
mechanically unrelated abilities.
Ie you can not benefit from more than one fighting style in a single
So for example:
If using Kung Fu 2 - Iron Skin which grants armour against bashing attacks,
you can not also do Boxing - Duck and Weave which lets you use the hight of your wits/dex for defence.
Similarly you also can not perform Boxing 1 - Body Blow which stuns targets if you do more damage then there size.
Because the hard tense muscles of the Kung-Fu Iron Skin stance leaves you without the flexibility to duck and weave fast enough to benifit from the boxing stance.
And unlike the Boxing stances, it is not designed to let you get enough power into a blow to stun a person with a Body Blow.
Basically these are serious martial arts.
A trainer has spent decades mastering the skills, mostly in competitions and demonstation fights. Many trainers wouldn't dream of polluting their are form by working out how to combine it.
There are exceptions, for things like Krav Maga, but those are specially developed combinations of techniques and are thus there own separate fighting style merits. (Which are still incompatible with the base techniques)