Rendering to different viewports (parts) of the same screen can be easily done with blending. As far as I know `GL_BLEND` or `glBlendFunc` isn't considered legacy.

For example splitting screen into four parts and rendering the same scene four times to each corner with different uniforms and different viewports:

    bindFramebuffer();
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    scene->setConstPerFrameUniforms();

    glEnable(GL_BLEND);
    glBlendFunc(GL_ONE, GL_ONE);

    //left bottom
    glViewport(0, 0, WindowWidth*0.5, WindowHeight*0.5);
    scene->setVarPerFrameUniforms(1);
    scene->draw();

    //right bottom
    glViewport(WindowWidth*0.5, 0, WindowWidth*0.5, WindowHeight*0.5);
    scene->setVarPerFrameUniforms(2);
    scene->draw();

    //left top
    glViewport(0, WindowHeight*0.5, WindowWidth*0.5, WindowHeight*0.5);
    scene->setVarPerFrameUniforms(3);
    scene->draw();

    //right top
    glViewport(WindowWidth*0.5, WindowHeight*0.5, WindowWidth*0.5, WindowHeight*0.5);
    scene->setVarPerFrameUniforms(4);
    scene->draw();

    glViewport(0, 0, WindowWidth, WindowHeight); //restore default
    glDisable(GL_BLEND);


This use `GL_ONE, GL_ONE` blend mode but for some effects like overlapping different modes can be used. [This][1] is nice visual representation of different modes

EDIT:

I thought about that and it can be done without blending. You just need four framebuffers or one with bunch of color attachments. To each of them you render your scene with different viewport enabled. Then in render pass to screen you bind those textures and blend them in more modern way using fragment shader. That's more complex but with more freedom of applying desired results.

  [1]: http://www.quake-1.com/docs/blending.jpg