Firebase transactions on two references possible?

Question

Firebase is providing the following example for transactions in its guide :

function toggleStar(postRef, uid) {
  postRef.transaction(function(post) {
    if (post) {
      if (post.stars && post.stars[uid]) {
        post.starCount--;
        post.stars[uid] = null;
      } else {
        post.starCount++;
        if (!post.stars) {
          post.stars = {};
        }
        post.stars[uid] = true;
      }
    }
    return post;
  });
}

So the object structure should look something like this:

posts
- postID
  - starCount: Int
  - stars
    - uid : Bool

I was wondering if I could still perform transactions, when the object structure would look like this:

posts
- postID
  - starCount: Int

postStars
- postID
  - uid : Bool

So it wouldn't be necessary to load all the userIDs of the users who've stared the post when you load the data to display the post.