THe \\ indicate that you have got a multi-value field.
You an handle (merge) such a filed to just one with an action of the type "Merge duplicate fields".
Please take care that the separator that you can specify in this action is not "\\" as this would lead to multi-value fields again as MP3tag interprets the \\ as field separator.
Once you have merged the fields you can manipulate the contents in all the ways you want to.