Could be as simple as using a white solid with an animated linear wipe on which a duplicate of the text is used as a track matte. Give that a try. If it's more complicated than that, screenshots may help to explain what you are trying to achieve.
The simplest solution is to duplicate your text layer and change the top duplicate text to white, add Set Matte to the White Text layer, and choose the Plane layer as the source. Parent the White text layer to the black text layer so when you move the black layer the white will follow.
You could also use a duplicate of the plane as a track matte for the white text layer but that adds a layer to your comp.