Make one video transparent over another video

I`m trying to make an interaction in which based on some parameters a colour grading or another will be displayed on a video. The way it is designed is to be a float value. Instead of one video or another, it`ll be a bit of one or another. Since I have the two files I`m trying to place one video over the other and just modify the alpha of the video on top... but when I place two separate video players on a camera if I modify the alpha of one, it also modifies the alpha of the other. Does any of you have any idea how could I make one video transparent over another video?