It might not be possible to book as one ways for the trip that involves a DL flight...

I always try to book on the internet, every time I call Flying Blue, I waste a considerable time on the phone, am faced with mostly unpolite and incompetent agents, and end up finding out I would have gotten a more satisfying result on line. And as much as possible, it is better to book one ways rather than return trips, because the more flights there is in you PNR the harder it is for the decerebrated pods who pick up the phones to understand what is happening.

10 years ago, calling Frequence Plus, you were talking with helpful, competent and flexible agents. Now forget it.