Complex sentence: The partner eye diverges from the affected eye during abduction, producing horizontal diplopia, in other words, if the affected eye is on the right, the patient will see "double" when looking to the left and the images will be side by side.
Split into simpler sentences: The partner eye diverges from the affected eye during abduction, producing horizontal diplopia. That is, if the right eye is affected the patient will "see double" when looking to the left and the images will be side by side.