We have seen Jasmine visiting with Rapunzel one evening instead of Snow. We have seen Ariel visiting with Cinderella instead of Aurora. I've seen Rapunzel by herself. A friend of mine said one evening Tiana was visiting with Rapunzel. It seems to happen more in the evenings.
)Seen pictures of Jasmine in with Rapunzel.
Saw Ariel in with Cinderella last week.
Heard stories from a friend of Mulan being brought over from Epcot to fill the spot (not sure how often that happens)
We've also been through the "line" twice in a row and seen Aurora both times, so sometimes you might have to wait 30-60 minutes to see the actual "mystery" princess if that's what your goal is!
I was at the MK on Friday 2/7 and when I went by, Snow White was on the sign with Rapunzel, but the sign on the Cinderella side showed Cindy and just a generic crown with the caption "Princess" for her friend. Did that mean Cinderella was alone?
Yesterday, it was Snow with Rapunzel and Aurora with Cindy.

Saw Ariel in with Cinderella last week.
Good point! Didn't there used to be a rule about characters not being two places at once? I know it doesn't apply anymore tho. Donald in dinoland and tusker house, Pooh & Tigger at the ride entrance and CP, belle in two dresses at Epcot...
They're more guidelines than actual rules.
When Snow White used to meet at the front of MK, she would always be gone when Dream Along with Mickey was happening at the Castle.Pixie Dust allows them to be anywhere, just not within "sight" lines of each otherWhen Snow White used to meet at the front of MK, she would always be gone when Dream Along with Mickey was happening at the Castle.