I heard the Tumko Mere Dil Ne video to understand the feedback posted earlier. And do I understand!
Their voices are not the perfect match, and the start and end of each syllable is not in perfect sync. For example "me - ri - aa - waaz - se". If they start the "me" out of sync but catch up together at the second half of the word at "ri", then it can be noticed. Especially when we have heard a pair who has every syllable in sync, even when they are in a situation where one is on stage and one is off stage!
Life is cruel. Fans compare you with the best that they have heard - what they (the fans) believe to be the best.