Okay, sorry if it seems like I dodged.
How do we decide when an exception happens: When the rules say so, such as spell or item descriptions that include such exceptions. For example a Hammer of Thunderbolts includes the exception that, when used with both a Girdle of Giant Strength and Gauntlets of Strength, the power of those two items will stack.
How do we decide when spells trump: Well, I'm not arguing about "trumping" effects. Clearly effects that contradict each other or can't logically co-exist won't co-exist. They hardly needed to write a rule to say that you can't be a lion and a snail at the same time. If you're asking when one spell effect from the same spell suppresses another, the answer is "Pretty much all the time". As in, unless there is a specific exception noted in the spell descriptor, the later casting suppresses the earlier one.
What do we base that decision on? The rules. If a spell, item, or spell-like ability says in the descriptor that it's an exception to the "no stacking" rule, then it is.
Does RAW provide an answer to that question? Yes. See above.
There are some effects that, if granted by different powers/spells/items will stack, even if the effects are similar. Anything granting a Dodge bonus to AC, for example. You can't stack two Haste spells, for example, because the bonus comes from the same spell, but you can add the Dodge bonus a Haste grants to a Dodge bonus from any other source. That's explicitly stated in the stacking rules. Same for "unnamed" bonuses. They stack as high as you like, so long as they aren't coming from repeated castings of the same spell.
Morale bonuses, however, don't stack, whether they come from a spell or a Bard's song. They aren't listed as exceptions, after all.
Is that any better?
