Thanks Simon,
Unfortunately I didn’t had time to work on that project for some weeks. I tried to follow your description in Motion4. But I didn’t got it working. To which layer do I have to apply the “link” behaviour. And to which parameters should it apply.
What I did:
I applied the link behaviour to my secondary text layer. Then I drag-dropped the clone layer of my primary text into the source box, applied the behaviour to the x-position of my secondary text and chose the simple border width as source-parameter. Next I saved the project as a template and tested it in FCP. Didn’t work.