Sorry, I wasn’t specific enough. You level up a costume where you have multiple heroes. You can switch which hero wears that copy of the costume. Now, if you have 1 maxed hero, and another on a lesser tier, it can’t wear that maxed costume. However, any max tier hero can wear it. Most importantly, Costumes no longer lock in War or Tourney defense.
See Q9 here:
In that same thread it talks about not needing duplicate costumes, which is true if you are using them separately in successive war attacks. If you are using 2 at once, you will need multiple costumes.