You should not have to adjust the focal length or move the camera.  I do not know what your rendered dimensions or aspect ration were, so this has to be an estimate. 

 

If you render to a 300 PPI then there will be 300 pixels for each inch. Knowing how large a new scene you need will give you an estimate of how large a scene you will have to have in relation to the 300 PPI. If your book cover was 6 by 9 inches at 300 PPI for a Vertical 3 x 4 aspect ratio, or 2400 x 2700 pixels and the new will be 12 by 9, plus another 6 by 9. Then adding all of this up will give you an idea of what the new aspect ratio should be. 

 

Or if uncertain you can increase the Aspect Ratio in steps until two sides of the area to render matches the new calculated area for the new cover, and then crop the remaining. 

As a rule when I do covers, I rarely do more than the top cover and spine.  The back is always reserved for the author's bio, pics, and the bar code for retail sales.  Maybe you could talk the person into a change and save yourself some work and give them more exposure with the author's backcover.

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