A slat if extended at a low angle of attack has very little effect on the CL of a wing ( slat increase CL max) but it would increase CD . However if you were to extend a trailing edge flap at the same value of alpha it would increase CL.

So if one flap was to extend and not the other there would be a large rolling moment produced. But if one slat was to remain extended there would be more drag on the wing causing the aircraft to yaw.