Thanks for your replies. 
So, he had one child then. As far as I know, his wife was around 11 years younger than he was and I'm surprised they didn't have more children. She was also the choice of bride by the Dauphine Maria Josepha for her son, the future Louis XVI, who married Marie Antoinette. I also read that he was eccentric and built this great palace (Karlsberg Palace) yet he wanted to be alone and hated people being around. Can anyone confirm his eccentricity and how was it manifested, if true?
Karl II August von Pfalz-Zweibrücken was a difficult character, this is what I remember reading about him. His parents didn't have a happy marriage. His uncle Herzog Christian IV von Zweibrücken thought that Karl was very stubborn and complicated. As Karl grew older there relationship became a bit better. After Christian IV died, Karl II August (as heir to Zweibrücken) refused to acceppt the morganetic marriage of Christian and his wife Marie of Forbach (Marianne Camasse).
Resently I read about a Caroline Augusta Freiin Gayling von Altheim (1748-1823), she was the longtime "grande maitresse" of Karl. As a young man Karl II August tried everything to marry her, because he loved her, but his relatives wouldn't allow it. He had her marry 1776 Ludwig Freiherr von Esebeck, later 'secretary of staat' of Zweibrücken. If you live in Zweibrücken his name should ring a bell. As of 1778 Caroline was the Obersthofmeisterin (Chief Lady in waiting in english ?!) to Herzogin Maria Amalia, the woman Karl had to marry. It was said that Karl respected Maria Amalia, but could not love her. Caroline had to stay in Karl and Maria Amalias Household, in later years she wanted to retire, but Karl refused to let her go.
If you can read german I would suggest to get the book: Der Herzog und die Tänzerin. Die merkwürdige Geschichte Christians IV von Zweibrücken. written by Prinz Adalbert von Bayern 1966. You should be able to purchase it through Eurobooks or Antiquario. If not you should go to a library and lend it through "fernleihe". Karl is discribed through the eyes of his uncle. The sad story of his mistress is from another source - Letters from Salome von Gelieu.
You also should get information about Karl in the Zweibrücker Stadtmuseum. But it could depent on the exhibition they have. I think they have more info on his father Friedrich Michael, younger brother of Christian.
Hope this gives you a little more information.