Original: Duke Ernest Augustus I of Saxe-Weimar, Wilhelm's second cousin, inherited Saxe-Eisenach; he and his successors ruled Saxe-Weimar and Saxe-Eisenach in personal union until 1809, when the duchies were formally merged into the Duchy of Saxe-Weimar-Eisenach.
Simplified: Ernst August I of Saxe-Weimar, Wilhelm's second cousin, inherited Saxe-Eisenach; he and his successors ruled Saxe-Weimar and Saxe-Eisenach in a personal union until 1809, when the duchies were formally merged into the Duchy of Saxe-Weimar-Eisenach.