Does anyone know if it is possible to book online, either at ba.com or on another site, two different fares on BA on the same PNR

So two people travelling, both on the same flights/cabins, but one at a slightly higher fare than the other. On BA.com if there is only one seat left in the slightly lower fare, and you try to book two people, it will book both in the higher fare. What I'd like to do it book one at the lower fare and one at the higher


Note this isn't about linking PNR's, its about booking both on the same PNR