Follow up after calling customer service:

I linked 2 cards for myself and had no trouble using the promos. I also linked 2 different cards in P2's entirely separate account. When I described this issue of the promos disappearing and it saying there was an error: "This promo has already been used by another account. Your credit card has not been charged." the rep told me it was because the promos "were already used by a related account". I asked what they meant by related since P2 is a completely different person, with a different last name, and 2 different Ink cards than myself. I was then told our accounts were "related by the same address" and that only one account per address could get the promos. This does not seem in line with what the promo says, but I'm not sure what to try from here.