It's really up to you and how you feel, but for me personally, I would do either the half ironman or the marathon, not both because one week won't be enough to fully recover and be ready for the next race.
I would caution against it, especially since you have not done a half ironman before, so you're not going to be quite sure how you're going to feel.

In your shoes I'd probably plan to do both, but then listen to the body when it came to the second one and have accepted ahead of time a slow day or maybe even skipping it. However, I know that I recovery pretty quickly most of the time.