It would be unwise to throw two untried bowlers into that situation against what is a strong South African batting lineup. There would be shades of the Indian batting scores when we had Bracken, Lee and that fat West Australian fella from a few years back.
Also, in that scenario Johnson is the 'senior' bowler. He's starting to find his rhythm and is living up to his potential, don't burden him with the extra roles straight away! Let him grow into himself as a bowler before you start with that sh*t...