Video guide:
https://www.youtube.com/watch?v=IxOd5toon44

How to install:
Just drag the Multisculpt .zip file into your viewport,
OR:
Open your preferences from Edit / Preferences
Under the Get Extensions tab, go to the upper right corner, press the down arrow button and choose "Install from Disk"
Navigate to the Multisculpt .zip file and select it. Check Enable on Install at the top right, then press Install from Disk at the bottom right. Save preferences if you want it to be loaded automatically when starting Blender.


How to use:
My preferred method for most operations is via the modified Modes Pie Menu. It’s enabled by default, its checkbox is located in Preferences/Add-ons/Multisculpt. There you can also choose your preferred pie menu variant.
Note: The default Modes Pie hotkey should be overridden, but if you have another custom ctrl+tab hotkey, you may need to resolve the keymap conflict manually.

The rest of the extension is located in the side panel (N-panel), under the tab "Multisculpt"


1. Select at least two mesh objects.
There's no need to filter your selection, any non-mesh objects will be filtered automatically.

2. Using the Modes Pie Menu, go into either Sculpt, Vertex Paint, Weight Paint, or Texture Paint Mode. 
Alternatively you can press Create Multisculpt from the N-Panel.
A proxy object is created from your selection and the original objects are automatically hidden. All modifiers, attributes, etc. will be preserved on your original objects,

The "Keep Common Modifiers" checkbox allows for any modifiers that are present on all objects to be propagated to the Multisculpt proxy object. This is primarily meant for Armature modifiers, but can be useful in some other cases. Just note that with some modifier types, like Mirror, Bevel, or Boolean for example, you may get unexpected results, as the modifiers get unified with the same parameters for the Multisculpt object (original modifiers will still be preserved and unaffected).

The "Use Current Multires Level" checkbox gives you the option to use the current Sculpt Mode Multires level on any objects with a Multires modifier, instead of their base geometry. All modifiers are still preserved as they are. All other objects that don't have Multires will still use their base geometry.
Note that when transferring back, the modifications are transferred to the same subdivision level, but not automatically applied to the base geometry. You can quickly do that for all objects by using the Apply Base All button.
Also Note that if color transfer is enabled, any objects that are using their multires level instead of the base geometry will be skipped for the color transfer step, as Blender doesn't support vertex colors on Multires subdivision levels.

3. Manipulate your proxy object
in any way you like, without changing the vertex count of the object. This means no extruding / beveling / deleting geometry, etc. It's also important to not change the vertex count of any of the original objects before transferring your multisculpt changes.

4. Using the Modes Pie Menu, go into Object Mode
(either once or twice if using Auto Fast or Auto menu variant), or go to Multisculpt Transfer if using Manual pie menu variant, or press Transfer Multisculpt in the N-Panel.
All your changes will be transferred back to the original objects. Modifiers, transformations, UVs, etc. are all preserved.
Note:If you want to transfer color or vertex group data, make sure that the checkbox in the N-panel is enabled!

Also note that when using the default Auto Fast pie menu variant you may lose sculpt undo steps when transferring from inside Sculpt Mode. If this bothers you, please switch to the Auto variant.

Color is transferred from the active color attribute of the Multisculpt object to the active color attribute of each original object. If the original object doesn’t already have a color attribute, it will be automatically created.
Vertex group data of each group is only transferred to objects that the group affects

Auto Clear Instancing checkbox will automatically make any of your original objects’ mesh datablocks single-user, so that they may receive unique changes.

4.Transformations may optionally be transferred to either a new shape key, the active shape key, or the basis.

You also have the option to compensate any deformations from existing (and currently active) shape keys.

Extra Buttons:
Isolate Multisculpt Object (in Modes Pie Menu, or N-Panel)
Press this button while you have at least one object selected, which has an existing Multisculpt object associated with it, or you have a Multisculpt object selected, and it will hide all original objects and show the Multisculpt object.

Hide Multisculpt Object (in Modes Pie Menu or N-Panel)
Hides the currently selected Multisculpt proxy object and shows all of its corresponding original objects.

Multires Apply Base All
Note: In version 2.0, this is done automatically whenever you Create Multisculpt with any objects that have a multires modifier.
This button will run the "Apply Base" command on any selected objects that have a Multires modifier. This is so that their base subdivision level has any multires displacement you may have previously sculpted. By doing this first, whenever you create a multisculpt proxy object, it will be the same shape as what you see in Multires.
Ideally, this should be done by default, but it's extremely slow. Thankfully, we have this separate one-off operation that we can perform whenever we need to. The button I've provided just does it automatically for all selected objects with a multires modifier. If any of the objects with multires have shape keys, they will be skipped, because the shape keys override the result from the apply base command anyway.

Should you encounter any bugs, or other issues, please get in touch and let me know so I can fix them!