Although I have no first hand experience of doing this, I don't think there is a way of doing it on BA.com. You could try ringing them and see if it can be done that way. You would still be in the same fare bucket, hopefully the lowest available providing two seats are available.

If this is possible and if you had wanted to sit together, I would be cautious for such a small saving as the seating situation may easily backfire with HBO allocations.