60000 + 48000 = 108000. I assume you actually did mean the 54000 + 48000 = 102000.

Irrespective, you can definitely book the tickets separately with 2 different PNRs, although there might be a risk of being seated separately if you can't do online checkin.

Alternatively, you could consider just buying the miles (i.e. not miles + cash) to get you to 108k, then book 2x 54,000. You might also have other sources of Avios, such as BA or Amex, that you could transfer across.

Although IB+ customer services aren't the greatest, you could also just ring them and book it over the phone.