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I think the issue might possibly be that you are trying to create 2 Identifiers simultaneously from a single source field. Identifiers is a complex metadata item, which is why it is not shown in the columns available in Preferences > Add your own columns. It is stored in the database with a key of both book ID and ID TYPE (e.g. isbn or issn or sudoc) and a result field of the value for that combination of book ID and ID TYPE. See the attached image of the metadata.db table "identifiers". That should give you a visual impression of the "destination" that you actually are attempting to populate.
You could create a temporary "Comments Short-Text Like Title" Custom Column each for sudoc and worldcat, then populate each using S&R separately. Then, use S&R for each temporary Custom Column to create a single Identifier for that respective temporary Custom Column. Once both of the Identifiers have been updated, you could delete the contents of the temporary Custom Columns.
Or, you could just keep the temporary Custom Columns permanently, and not make them into Identifiers. Up to you.
Note that if you want to see an Identifier in the Library View, you must create a Composite Custom Column (i.e., built from other columns) with a template such as {identifiers:select(sudoc)} or {identifiers:select(worldcat)} or {identifiers:select(isbn)}.
If you already have a "real" Custom Column for sudoc and worldcat, you would not need a Composite Custom Column to see their values. Your choice.
As usual with Calibre, there are many ways to do something. Your personal preferences should guide you to the correct one for you.
DaltonST
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