Kakifrucht Yes, all missing translations for formal variant are taken from informal translations, so there is no need to translate everything twice - you can focus on informal translations and bot will handle duplication in language pack repository.
The only real downside of this is that Weblate will treat these skipped translations as missing, and all status badges will show that extensions are mostly untranslated, even if translations are actually complete and no action is required. I have no idea how to improve this - the best way to signalize that no action is required for specific phrase, is to translate it.
Hiding status badges for formal translations in readme is also an option, but I'm afraid that it will be to hard for users to find how to contribute translations if they're using formal variant.