Looking over the items I'm going to want for my character, I find myself wanting two neck slot items (an Amulet of Mighty Fists and a Necklace of Ki Serenity).
The Hand of Glory allows a character to wear an extra ring by taking up third neck slot, are there any other items that exchange what slot can be used where? (Hopefully one to let me wear more than one necklace at a time?)
There are a few magic items that will use a slot different than normal, to name a few:
But no, there are no magic items that will grant you an extra neck slot.
Non-humanoid creatures have access to the Extra Item Slot feat, that grants an additional magic item slot. But this would only help you if you had no neck slot, since the feat can only grant you a slot that you don't have already.
Another option would be to use a Magical Tattoo, which can be also placed on your neck but does not take up your neck slot. Or even take a look on the available Ioun Stones to check if the effect you want is not already covered by one of them.
Considering you are trying to obtain extra ki using the Necklace of Ki Serenity, you should take a look at the Ring of Ki Mastery, which allows you to store up to 2 ki points for later use and reduces the ki points used by ki abilities.
You can combine items into a single item. The item-creation guidelines generally recommend that effects after the first (most expensive) should cost 50% more than normal. So for an amulet of mighty fists that is also a necklace of ki serenity, which one is most expensive depends on the enhancement bonus of the amulet of mighty fists: +1, and the amulet is cheaper, +3, +4, or +5 and the amulet is more expensive (they are tied at +2).
So we end up with this:
\begin{array}{c|c} \textit{Amulet of mighty fists} & \textit{Amulet of mighty fists} & \textit{Necklace of ki serenity} & \text{Total} \\ \text{enhancement bonus} & \text{Cost} & \text{Cost} & \text{Cost} \\ \hline +1 & 4\,000\text{ gp}\times150\% & 16\,000\text{ gp} & 22\,000\text{ gp} \\ +2 & 16\,000\text{ gp}\times150\% & 16\,000\text{ gp} & 40\,000\text{ gp} \\ +3 & 36\,000\text{ gp} & 16\,000\text{ gp}\times150\% & 60\,000\text{ gp} \\ +4 & 64\,000\text{ gp} & 16\,000\text{ gp}\times150\% & 88\,000\text{ gp} \\ +5 & 100\,000\text{ gp} & 16\,000\text{ gp}\times150\% & 124\,000\text{ gp} \\ \end{array}
And the combined item uses a single neck slot.
However, that cost is preposterous: both amulet of mighty fists and necklace of ki serenity are rather overpriced for what they do in the first place, and the 50% premium is just adding insult to injury—there are cases where it’s appropriate, maybe, but this isn’t one of them. So I would just ask your GM for a houserule, either reducing the price of amulet of mighty fists, necklace of ki serenity, or both, eliminating the 50% premium for combining, allowing those effects on items in different item slots, or something along those lines. If your GM won’t allow that, I strongly suggest giving up on the necklace of ki serenity; it isn’t that good. Actually, I’d strongly reconsider going for any kind of unarmed build—unarmed strikes are woefully underpowered, and if your GM isn’t willing to work with you to fix that, my opinion is that it’s a needlessly painful process to try to fight Pathfinder on it.
Ten-Ring Sword (Ultimate Equipment p. 162) gives you an extra ring slot.
I haven't seen anything in Pathfinder that gives an extra neck slot. Short of growing an extra head, your best bet might just be to work around the restriction via custom crafting, e.g.:
Your options depend on your GM more than the rules as written when it comes to this, given that no such items exist. Even with custom magic item crafting, a mechanic built into the game, the GM has the final say on whether an item is broken or not. Here's the options that you would have to work out with your GM if you want those two items.
If your GM is fully supportive of custom magic item crafting taken to its extreme within RAW and your character doesn't need one of their other slots (perhaps ring, chest, or head, depending on your build) and you have access to a magic item crafter (I'm assuming you can't craft this on your own), then you could simply hire the crafter to craft an item with the same properties as one of those two items in a different slot. As a GM, I could see the Wrists slot being a reasonable substitute for the Amulet of Mighty Fists, given that it would clash with the Bracers of Armor, just as the classic conflict for a Monk's Neck slot would be Amulet of Mighty Fists vs. Amulet of Natural Armor.
If your GM is familiar with some of the shenanigans that can be pulled via Craft Wondrous Item, he might disallow changing the slot, as is recommended by the rules for magical item crafting. You could have the abilities of one item crafted into the other, which is allowable under RAW and provided for under magical item crafting. As a GM, I'd be okay with this, due to the extremely high gold costs involved, which limits your resources that can be used for other magic items.
Similar to Ioun Stones, you might be able to make a "charm" of some kind that lets you have both effects. An easy option would be a magical tattoo that grants you the full benefit of one item or the other. More expensive than even combined effects, but still possible.
It's possible that your GM would rather let you craft a custom magic item that allows you to substitute one slot for a neck slot, similar to to the Hand of Glory. You could theoretically have one crafted, but this is the likely option for GMs who don't like magic item crafting but are willing to work with their players to make sure they get the items they need. As a GM, trading one slot for another is an established precedent in the game, with the most common slots traded around being ring, body, and neck, so this also seems reasonable, assuming the other slot is a ring or the body.
Sometimes, GMs don't allow custom magic items, either by your hand or theirs. If this is the case, hopefully they're fair about it and aren't lowering the hammer on your dreams but letting someone else have their unique doodad.
