We had one before with Thomas Cook at UK airport . Same thing but our hand baggage weights were 7kg and no hand baggage. When I pointed that made us 3.5kg average each, she still insisted one of us was over and we needed to adjust things before we were getting checked in.
So we held up the whole queue putting half of it into a carrier bag we luckily had in the hand luggage bag. She weighed both, gave us our boarding cards, then watched as we put the carrier bag into the other bag, right in front of her, and walk away.