My Dual Flush Buttons both depress together?

The plumber fitted a new toilet cisten for me which has a dual button mechanism in the centre of the lid. The buttons have a cable connecting them to the filling mechanism in the tank. Whenever I press on one of these buttons both of them depress defeating the purpose of having two separate buttons in the first place. The amount of water that flushes depends on how long I hold the button down rather than what button I press. Does anyone have any idea how to fix this. Any help appreciated