You can not simply use 2 materials on same part of the mesh.
You can have 2 materials assigned to different parts of the mesh like in blender (material ID, Vertex ID)
I understand that you send me the mesh in discord but that will not solve your problem. How you plan to change your text ?
you can assign vertexes to materials like below in blender
however as said this is no way going to help you achieve the task you want.
You can assign a single plane mesh from engine and use the nodes i gave you the shortest way possible.
If your supervisor insists to do with a static mesh and create text on it without geometry script or materials tech, all of the forum will be interested in that i am pretty sure 