How do I add UV shells from one UV set to another?

[vordermand]

I added some geometry to a mesh, which created a new UV set with the shells of that new geometry. The UV set that contains the new UV shells is called "map 1," I want to copy or move its shells into the UV set called "Red Wood," but I don't want the shells that are in "Red Wood" to disappear!

I have tried to right click and "Copy UVs to UV set" like this:

But this causes the old shells inside the "Red Wood" UV set to disappear, I don't want that, that's the issue I'm having. So in short: I want to put the UV shells of one UV set into another one without making the UV shells in the other UV set disappear.

For additional information, although this may be irrelevant, hovering my mouse over "Copy UVs to UV Set" causes this red error message to appear in the log:

// Error: file: D:/Program Files/Autodesk/Maya2019/scripts/others/buildCopyUVsToUVSetMenu.mel line 149: Object's name 'uvTkUVSetTreeCopyUVsToUVSetObjectUVSet3' is not unique.

[mspeer]

Hi!

Select only the particular UVs you want to add and use "Copy UVs to UV Set".

(You can only add UVs that do currently not exist in the other UV Set, already existing UVs for particular components will be replaced).

[vordermand]

That's not what I want though. I want all UV shells that are going to share the same material (hence "Red Wood," that's the material) to be in the same UV set. One way to do that is to delete one of the UV sets, and create new UVs for the geometry that now no longer has any UVs, that's what I ended up doing. I had to double-click select the faces of all those individual components in the viewport first and create UVs for them, because they didn't have those anymore after I deleted their UV set, each individual face had all its edges cut and those UV shells were invisible in the UV Editor. In my mind it should've been as simple as Ctrl C + Ctrl V (I actually tried doing that as well but it didn't work). There is no better way to do it? In that case, because I have a different UV set for each material that the building is going to have, is this not a good and common practice and should I do something else instead?