I don't care what anyone says, but an Xtina and Britney duet would break the internet, break records and inspire people for world peace.
Most ICONIC Rivalry/Friendship in Music history. CHECK
POP Royalty endorsed by Madonna, Michael, Janet, Whitney etc CHECK
Iconic videos, songs and GLOBAL trendsetters when they were teens CHECK
All the current pop girls - Beyonce, Taylor, Selena, Miley, Gaga have cited one or both as inspiration. CHECK
Given their history I think a duet is possible under the right circumstances. This video alone proves how thirsty people all over the world are for ANYTHING Xtina-Britney - even after all these years. All they need is each other for a #1 world smash, perhaps #1 song of all time is possible too.