Copying UVs from one instance to another?

I have two instances of the same shape; they have the same vertex order and are identical except for the UV coordinates. Is there a way to copy the UV coordinates from one into the other, as a secondary set of UVs? Sort of like “join as shapes” only with UVs?

Yes there is a way to do that, from selected to active: CTRL-L -> Transfer UV Maps. Hopefully, it is what you’re looking for.