I just got back from WDW after a week, and can answer that for you. Our 6 year old nabbed plenty of autographs, and photo-ops with tons of princesses.
The building you speak of across from the carousel houses 2 wings so to speak. The side with Anna & Elsa also has a random princess included. The Frozen sisters are together, and in the same room at the time was Princess Aurora.
So you basically get 3 autographs (Anna & Elsa sign on the same page), and 2 photo ops.
In the same building...the other entrance leads to 2 other random princesses. I've seen Snow White, Cinderella, and Rapunzel there (random).
Anecdote: My wife and I goofed, and chose a FP+ for Anna & Elsa, but for next week by mistake.
When we scanned our Magic Bands at the entrance the CM tells us we dont have a FastPass for the day. I check my cell phone, and notice we didnt do Thursday the 5th..we did Thursday the 12th. Our hearts sank as our daughter was so excited to go in. The Standby time was 210 minutes.
My wife refused to quit, and decided she would make the wait. I stood outside despondent, and still upset at our FP+ goof. I looked up, and said a little prayer for some Disney Magic hoping something would give. So I get some drinks, and go inside to make the wait with my wife and daughter. Can you believe it only took us 39 minutes?!!! We were stunned, but so damn happy!!!!

Disney Magic indeed. We still cannot believe how we lucked out!
I dont know what was up with the wait, because I have seen some murderously long lines extending outside. We did this at 4PM on thursday FYI.