In many cases you have 2 very different sets of fans, ratings for RAW if I remember correctly didn't massively jump up by 2.0 when WCW went under (before the quality of the WWE product slipped), obviously different nights could make a difference because there will be fans that watch both, but one thing that pushed the ratings to their heights was the competition, the WAR, the who do you watch, each show trying to out do the other, on seperate nights you just don't get that feel.