Similar thing happend ot Ozjet. 2 aircraft on a charter. 1 aeroplane to carry ~ 104 pax and the other ~ 70 pax. The aircraft with 70 pax was fuelled to depart with tankering fuel assuming the 70 pax and bags.
Each pax group was loaded onto the wrong aircraft. The pilots on the aircraft carrying tankering fuel wondered why so many pax were getting on and asked the ground agent how many were being loaded. When the Captain was informed that 104 pax were expected, he ordered that boarding be discontinued and pax on board counted. It was determined that the aircraft at that stage was 2kg below max ramp weight...