I'm assuming "user set" means the user bank and you are not talking about Live Set.

There are not multiple user banks. There is just one. You mention loading something into a 2nd "user set". Again, assuming this isn't the Live Set -- there's just one User Bank which is a continuous pool and no segmentation from a storage perspective. There are, however, 8 different Libraries. This is probably what you want to leverage.

Any X7U file you can load directly into the library area. Chances are you have one free. If you do not, you may have to delete a library that you can easily restore because you have the X7L file - or generate one. That's a 3-to-4 step process: 1) Backup your user area (if you don't have one of the current one) 2) Import the library you want to delete to the user area -- it can't be promo material or apparently the waveforms won't copy 3) Save the user area as a library file. Note: if you know the original library had waveforms -- and the save to library only takes a second or two -- you can pretty much determine that the waveforms were not imported and will not be contained in this X7L file. That's not a given, just one rule-of-thumb type observation that doesn't necessarily account for all cases.

... assuming you have that free slot squared away, you load the X7U (from User Backup C) into the Library -- then you use Library Import to move any single Performance from Backup C (or group of Performances) into the user area. This "Library Import" does not overwrite the user area -- it simply appends to it with no overwriting.

You can finish up by deleting the library which holds the contents of User Backup C, because you've imported the Performance(s) you wanted into your user area.

If your firmware is too early, "Library Import" may not be a available feature. Certainly the latest firmware (and many earlier ones as well) have this feature.

Moving a single Performance does not have to be complicated at all. Here is the background you need...

What is troubling is your terminology when referring to some of the features/functions. Which prevents anyone from giving you the answer you seek... here’s why:

There is no Performance USER Bank 1 and Performance USER Bank 2 - there is just one User Bank of 640 Performances. What maybe initially confusing, is how the words “User” and “Bank” and “set” get thrown around...

You do use the word “set” (but never the correct term “Live Set” - so it is very unclear what you are talking about, exactly. Are you referring to the User Banks of the “Live Set” feature?

A “Live Set” is a collection of Performance titles placed in a specific order... they are also referred to as being placed in User Banks because they allow you to easily place your creations in your own customized select order (designed for use on-stage when gigging).... they reference (‘point to’) locations elsewhere in the instrument and are simply made up of existing Performances.

Also troubling is no mention of any Waveform data (as managing data with custom Waveforms may require special attention, depending on your personal workflow). Below we’ll describe a way to manage your Waveform data to maximize flexibility.

When you load a User File (.X7U) you will overwrite all data currently in the User Bank. This is true.
By the way, a USER File .X7U can only be loaded to your User area (never directly as a Library)@Jason (compatible legacy file types can be loaded to a Library, almost anything but a .X7U User File can be loaded directly to a Library... just FYI).

Now, if you are seeing the words “User 1” and “User 2” etc.,, you must be referring to the “LIVE SET” User Banks, there are eight of these “Live Set User Banks”... each has 16 Pages with 16 slots (256 titles) a total of 2048 slots.

Okay, Why 2048 slots for 640 Performances? It allows for *repeats* lots of repeats! Say in the first song you need acoustic piano, then strings, then acoustic piano, then brass, then strings and finally back to acoustic piano... simply enter these in order in your Live Set and advance through them with one click, touch, or pedal step. For those with specific set lists this allows you to simply advance through your Performances.

When you load a User File (.X7U) you will overwrite the current User Live Set Banks, as well.

Also troubling in your post is the lack of the word “Library” which leads me to believe, perhaps you’ve never installed any data to a Library.
Library, might be the key to your solution (are you using the Library feature?)

A Library is basically a copy of the entire USER BANK. A Library is never overwritten by loading something... it will remain in memory until you manually DELETE it, or you “initialize all data”.
Significantly, each Library includes up to the following maximums:
640 Performances
2048 Waveforms
2048 Live Set slots
256 Arpeggios
256 Motion Sequences
(128 Audition Phrases)
32 Curves
8 MicroTunings

When you create a LIBRARY File all of the above data is included in the .X7L File... which can then be burned into your instruments Library area (Flash ROM) into one of eight semi-permanent memory locations (they will remain in your instrument and accessible at any time).
You create a Library File literally by SAVING your current User Bank AS a “Library File”. [UTILITY] > “Contents” > “ Store/Save” > “Content Type = Library File”

So we need clarification from you on the terminology (before we can have a conversation about how to proceed):
1) User Bank of Performances accessible by pressing [PERFORMANCE (HOME)] > [CATEGORY SEARCH] > Set Bank = User.
2) User Bank within the “Live Set”.... accessible by pressing [LIVE SET] > in the upper left corner Bank = USER 1-8; Page = 1-16

Typically, the items in your “Live Set” reference Performances that are currently accessible. Typically, they would reference the current Performance User data (or a Factory Preset) - which is listed in the order they were added. The “Live Set” is simply a re-listing in the specific order *you* want to use them.
So that when you load a new .X7U File, you overwrite all previous User Performances and the previous Live Set.

Move one Performance
If you need to move just one Performance from one File and you wish to copy it so it appears in both files... then all you need do is the following:
Call up the Performance you wish to move.
Press [UTILITY]
Touch “Contents” > “Load”
Set the “Content Type” = User File
Select the File to which you want to add the Performance... this will load the data in that file, overwriting all current User Bank data except what is in the current Edit Buffer.
Press [HOME]
The Performance you wish to move is still in the current Edit Buffer... you can press [STORE] to “Store as new Performance” this will add it to the current, just loaded, User Bank of Performances... it gets stored in the lowest numbered empty location. (if you now wish to include this Performance in a Live Set, do so, as you normally would. Recall the Performance Home screen, Press [SHIFT] + [LIVE SET] to register it to the current Live Set User Bank.

Finally, you must overwrite the .X7U File you most recently loaded... doing so will update it, *adding* your new Performance and save the fact that you included it in a Live Set.

Alternative Methods to Move one Performance:
Use the MONTAGE CONNECT computer application to capture and store your Performances as Single Files. This allows you to quickly and easily transfer a Performance between loaded files. Highly recommend: if you take the time to create a Performance storing it as a .X7B (bulk) on your computer is the ultimate backup.
If you’re wondering, does it deal with custom Waveforms? Well, this depends on how you use the 1.8GB of Flash. If you use it to keep permanent reference locations for your Waveform data, then using MONTAGE CONNECT to back up and move individual Performances without issue. Because as long as it is referencing *your* instrument’s Waveform List, it will always ‘find’ the correct data.

Use the Soundmondo app for iOS devices to bulk your individual Performances from the MONTAGE: like the MONTAGE CONNECT program, you can keep individual copies of your Performances - which can be bulked from your device to the instrument’s Edit Buffer... making moving single Performances around a breeze. Again, because your Waveforms remain in their Flash locations, as long as you do not alter your instrument’s Waveform list, your bulks will always find their required data.
Hope you can decipher that.

Extra Credit:
I usually include a note here about the importance of the correct terminology (as it is) even if you think it awkward, it’s what we have to communicate back and forth. The word “User” and the word “Bank” have specific uses in the synth.

In general, those things done at the Factory are called “Preset”. Those items that you customize are “User”. The User Performance Bank is filled from the lowest numbered empty location upward, 1-640. The User Banks of the Live Set can be thought of as a ‘your choice’ listing of your Performances. Instead of just going to the lowest numbered vacant spot, YOU choose the order. It’s just a listing. Technically speaking, Library data is a kind of “User” - but it gets a special name as it is user data that is not overwritten by a Load operation. It is semi-permanent user data. (User definable presets).

When you press [CATEGORY SEARCH], from the Performance HOME screen, you see a configurable listing of your instrument’s Performances.
If you set the “Bank = User” you will see a listing of current loaded User Performances... you can access and play them from this listing;
you can re-order them alphabetically by Name (or reverse), or the Default is 1-640 (without any gaps for deleted items), or by Date (or reverse).

Wouldn’t it be nice... to have a custom listing of the current Performances in the order you would need them when performing on stage? wouldn’t that be nice? Well, that’s the raison d’être for the “Live Set” feature. You now know the reason behind the feature. A user created listing of the programs... your sounds, listed your way.

Hope that helps.