Female duets rarely work well unless there is a distinct contrast between the two like Janet and Missy. There are some exceptions, Mariah and Whitney, Brandy and Monica but for the most part it doesn’t work. That is the reason why there are so many male/female duets because there is a natural vocal contrast. I guess it is also the reason why there aren’t many girl band duos out there, if it did work Beyoncé and Kelly could have gone it alone and wouldn’t have had to replace people.