You would be relying on the discretion/benevolence of the individual checkin staff member - maybe you get lucky, maybe you don't. Personally I would split out the excess 9 kg as it's much easier to sort these things ahead of getting to the airport. Even buying a cheap small suitcase would be less cost than the excess baggage charge, but either way it seems crazy to risk incurring extra cost when you have already paid for much larger luggage allowance than you're using (albeit divided differently).