Flying Blue (Air France & KLM) - Different classes of service on same PNR?

Urgent question as I have to be at the airport in 3.5 hrs.

The 2 of us are travelling on one NWA.com-sold KL metal locator. One BC seat has come free and I'm hoping to UG using miles at the airport. If only that one seat is available, would it be possible to UG just one pax on the PNR or would it have to be the both of us or nothing? Just wondering whether I should split the PNR now or later.

Thanks for speedy replies

PNR would have to be split.

PNR would have to be split.

Confirming this is the case.