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).
Best Answer
In this case it makes more sense to modify the effect than it does to try and stack multiple copies of itself. Boots of Speed are a CL10 item and they offer ten rounds of haste per day because Haste cast at CL10 has a duration of ten rounds. Make them a CL20 item, and that'd translate to 20 rounds of haste instead.
When it comes to the price, the working for the original item is:
Spell Level (3)
* Caster Level (10)
* 2,000gp (use-activated effect)
/ 5 ("1" charge per day)
= 12,000gp
If you make that CL20 instead, you increase the base cost to 24,000gp, or 12k to craft. You could achieve the same effect for the same cost by calling it two charges per day for a total of 20 rounds, instead - but making the item CL20 instead increases its resistance to magical interference by others.
I am guessing your character is not a 20th level caster. Luckily, as per the sidebar on the SRD, no part of the magic item crafting rules actually requires the crafter to meet an item's CL; you can create your item at whatever CL you like, as long as it's high enough to cast the required spells and you can afford the cost.