各テクスチャを異なるテクスチャ ユニットにバインドしてから、マルチテクスチャ 座標を使用する必要があります。次のようになります (既にテクスチャがあると仮定します)。
glActiveTexture (GL_TEXTURE0); // First texture is going into texture unit 0
glBindTexture (GL_TEXTURE_2D, tex0);
glEnable (GL_TEXTURE_2D);
glActiveTexture (GL_TEXTURE1); // Second texture is going into texture unit 1
glBindTexture (GL_TEXTURE2D, tex1);
glEnable (GL_TEXTURE_2D);
glUseProgram (yourGLSLProgramID);
glUniform1i (sampler1Location, 0); // tell glsl that sampler 1 is sampling the texture in texture unit 0
glUniform1i (sampler2Location, 1); // tell glsl that sampler 2 is sampling the texture in texture unit 1
///... set the rest of your uniforms...
glBegin (GL_QUADS);
glMultiTexCoord2f(GL_TEXTURE0, 0.0, 0.0);
glMultiTexCoord2f(GL_TEXTURE1, 0.0, 0.0);
glVertexCoord2f(0.0, 0.0);
glMultiTexCoord2f(GL_TEXTURE0, 1.0, 0.0);
glMultiTexCoord2f(GL_TEXTURE1, 1.0, 0.0);
glVertexCoord2f(width, 0.0);
glMultiTexCoord2f(GL_TEXTURE0, 1.0, 1.0);
glMultiTexCoord2f(GL_TEXTURE1, 1.0, 1.0);
glVertexCoord2f(width, height);
glMultiTexCoord2f(GL_TEXTURE0, 0.0, 1.0);
glMultiTexCoord2f(GL_TEXTURE1, 0.0, 1.0);
glVertexCoord2f(0.0, height);
glEnd();