In Pathfinder, the wondrous item crafting rules say that you can put additional effects onto a single item of a particular item slot at an additional 50% cost. I've also heard it said that weapons can have (for example) multiple flaming enchantments, each of which proc for 1d6 fire damage on a hit, so there doesn't appear to be a general rule against duplicate magical effects on a single item.
My query is: can I have a single feet-slot item empowered with two Boots of Speed enchantments and expect any sort of improved performance?
I understand that haste effects do not stack. I'm wondering if I could activate those enchantments separately, effectively providing twenty rounds of haste per day instead of the usual ten. Or would both instances always activate at the same time, giving me the usual ten rounds of haste and a 9000gp-shaped hole in my pocket?
I know I could just carry an extra pair with me. But so long as Paizo refuses to include the indispensable Greaves of Automatic Shoe-Swapping +1 in any upcoming splatbooks, that seems appallingly inelegant (and inconvenient for expensively piling on additional effects).