Do you realise that the 10 kg object will move up the slope and the 20 kg object will move downwards?
Have you resolved (in directions parallel and perpendicular to the slope) the forces acting on the 10 kg mass?
Do you realise that the magnitude of the acceleration for each object is the same? The 20 kg object moves downwards with an acceleration of a m/s^2. The 10 kg object has an acceleration of a m/s^2 up the slope and an acceleration of 0 m/s^2 perpendicular to the slope.
Have you applied Newtons Laws of Motion to each object (for the 10 kg object you apply the Laws in the two directions)?