Creating a new tag from existing tag with multiple entries

I used a format value action in an attempt to create a tag from other existing tags that have multiple entries and it seems mp3tag ignores all but the first tag entry when running the action. eg

Format value NewTag using %genre%\%style% using the following as input:

genre=Pop
style=acoustic
style=alternative

creates a NewTag with Pop\\Acoustic as the entry... alternative gets dropped.

is there a way around this so that all values ascribed to a tag are correctly appended?

Solved it - it's necessary to merge duplicate fields first.

You can also use $meta(style) or $meta_sep(style,\\)