I just went through something kind of similar with Delta on our next flight into STT. Everyone in the family had enough miles to get one leg free. So I called Delta and asked what to do. They said I had to book everything separately. So I had to get separate tickets for all 4 of us by using everyone's own skymiles account. Luckily I was still able to pick all our seats next to each other. Then the return I had to book a separate trip.
I know it's not your exact situation but I think the takeaway is that you'll have to book your legs separate.
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