I did this recently, we had OBB from a pre-pandemic cruise and FCC from a pandemic cancelled cruise. Initially it was 3 of us in one stateroom and I wanted to book two staterooms on a new cruise. The OBB was only valid for one cabin, the second one was booked a full price and everyone's individual FCC was applied to them on the new booking. It worked out that one stateroom was paid in full and had FCC left over (new cruise was cheaper) and the second stateroom had a balance owed as we'd added an extra person.