Shader needed for Augmented reality, Unity 3D, video transparency and mask

We need shader which works on Unity 3D material assigned to RawImage on the Canvas, which reads the right half and renders only the left half according to alpha from right while playing the MovieTexture ( video ). We need it for our Unity project with Augmented reality video. Unity 3D does not handle transparent video properly. The initial workaround is to render original video and its mask simultaneously composing the result in a shader which renders transparency according to mask. However, this is not optimal since Unity renders/decodes two videos at the same time and mask lags behind the original producing wrong mask for the current original frame We need that shader plus: 1] only one video with double width containing both key frames and mask - see attached image. We will have this video for testing. Need only shader or app for desktop augmented reality that can play more videos.