Most airlines will let you share weight but you still cannot have a single bag over a specific weight (I think 23kg or 25kg) depending on airline and country of departure.
Some low cost airlines don't even let you do that with family members. If there are two people travelling, each with say 22kg allowance (total 44kg), and one case is 23kg and one 17kg (total 40kg), they will charge for the bag 1kg overweight.