This is somehow causing errors with bone constraints
So I would like to “merge” these two data blocks back into “Armature”, but for example copying and pasting the bones doesn’t seem to work (it just creates a new datablock instead of replacing the bones in “Armature”)
I’m using Blender 4.02
Yes thanks for that info, but these are in a single armature object. So I have the “Armature” object which has these two data blocks. It makes no sense at all!
It might be that I don’t know what I’m looking at but it looks like a bug of some sort, since I didn’t do this intentionally, and I wouldn’t know how to be honest.
Additionally there’s no way to copy the bones from one data block to the other, so it can’t be fixed that way apparently, and that’s why I’d like to be able to merge the two data blocks.
Following on from @_hris, that selection box is for choosing which data block the armature is using.
If you want to Ctrl-J them together you need a second Armature object to join with the first one.
Create a new Armature, then in it’s data block, you can select the Armature.002 data.
In your first Armature select the Armature data block. You can ignore the new armature data block that will have been created by the new Armature.