OpenGL merge multiple framebuffer

Question

I have a 2D scene splitted in multiple z levels. For every z level i got a framebuffer. How can i merge these framebuffer into a final framebuffer, which will get displayed (whole scene)

The idea behind this: most animation (etc) will happen in higher z levels. with seperate framebuffers i could just render z level with changes so im not ending up to redraw backgrounds.

Answer 1

If those framebuffers are backed by textures as color attachment, just draw those textures full viewport back to front with depth testing and writed disabled and blending enabled.