Complex sentence: Both provide a portion of the lift, although they are not able to produce twice as much lift as a single wing of similar size and shape because the upper and the lower are working on nearly the same portion of the atmosphere.
Split into simpler sentences: Both provide a portion of the lift, although they are not able to produce twice as much lift as a single wing of similar planform. In the case of the biplane, the upper and the lower are working on nearly the same portion of the atmosphere.