FWIW -this is one reason that I will frequently book each of us on individual itineraries (or when DD was younger, I would put her with one of us, but do DH separately). Then what I would do is book the ding (and pay for it), then cancel the original, then use those credits to book the next ding. Of course, we go 2-3 times a years, so I've always been able to use up the end resulting credit.
Sometimes is also worthwhile to see what would happen if you wanted book 8 seats with the ding. Sometimes that give you an idea of "available" it really is.
There's have been times with the separate itineraries, that I've been able to get the ding rates for some but not for all.
Anne