Complex sentence: Frank Borman on "Apollo 8" and Rusty Schweickart on "Apollo 9" both experienced identifiable and reasonably severe cases, in the latter case causing the mission plan to be modified.
Split into simpler sentences: Frank Borman on "Apollo 8" and Rusty Schweickart on "Apollo 9". Both experienced identifiable and reasonably severe symptoms--in the latter case causing the mission plan to be modified.