We did get to see Applejack take Rarity's place a bit in "Magical Mystery Cure", but I'd love to see both of them taking on each other's lives. They have such different careers and would struggle at each other's jobs! I guess their main similarity when it comes to responsibility is that they both have a younger sister, whom they look after from time to time, but otherwise their day to day activities are probably very different.

That would be funny.

I laughed in Simple ways when Rarity struggled how to work a plow.

This would be fun to watch.

although Rarity and AJ sound fun, but their differences have been explored a lot in the show so I vote for a different pairing.

There's a story like that "the OL' swtcharoo"
I wouldnt be the one to recommended it though, it's a dirty story