Complex sentence: In the U.S., the episode remains the highest rated episode of the series, in total viewers, although the episode "SeinfeldVision" currently holds the title of the title of highest rated episode of the series in the key adults 18--49 demographic.
Split into simpler sentences: In the U.S., the episode remains the highest rated episode of the series in total viewers. The episode "SeinfeldVision" holds the title of the most viewed episode in the adults 18--49 demographic.