Hi Jan,
You are probably asking this because if you enable 'owner' you'd only see the owner login name as opposed to the firstname / lastname which makes it not very usable right?
whereas for the same ticket in the overview, if you enable it, you DO see firstname and lastname:
This is a bug; and to answer your question, you'd first need to look up the user first and last name before you can use it in the DTL template.