Most airlines let you share your allowances between family members, but BA have always been a bit anal retentive about following the last dotted i and crossed t of the rules. The solution is simple: put the bags on the scales before leaving home, and if the total is under 46 kg, move stuff from one bag until each bag is under 23 kg, even if it means having feminine underwear in your bag or shaving kit in your teenage daughters' stuff.