Boosts of the same kind don't stack.
For example, if you equipped the Cape of Awe (25% XP, CP, Gold, Rep) and then the Dark Summoner armour (15% Rep), you would then have a 15% Rep and 25% XP, CP, Gold overall boost.
The Dark Summoner's Rep boost replaces the Cape of Awe's because it was equipped after, even though it's a smaller amount.
However, consumable boosts, server boosts, and gear boosts will all stack with each other, even if they're the same kind. For example, you could mix a 48hr XP server boost, a 1hr XP consumable boost, and the Cape of Awe. This would be the best time to farm if you can align all three.