compositing cna be na art itself :)
i've done it for years as often you'll find small errors, or things that need tweaked etc etc
so I may render a small area with a change to a material, and layer that over original image and so on.

as Bruno notes, however you could lose shadows, so try rendering the left side of an image, and the right side seperately and composite them?
also, optimizing objects and textures helps a lot
so does setting display options to show only wireframes with no textures applied

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