Obviously one occurs when Harry *spoiler* dies and goes to Kings Cross Station.
The other is the horcrux version of Harry and Hermione, so that makes a lot sense because what kind of clothes would two representations of voldemort's soul be wearing. It would be cheesy if they put them in all black or something lame emo getup.
The dancing scene sounds very sweet and I'm sure it will only add to Ron's jealously and make the locket scene more effective. Plus Harry and Hermione are tight like that so of course they would dance together.