I, personally, don't have experience with this but I thought I would mention that a friend of mine had her second when her first was only 23 months. Her first was in a toddler bed a few months before so that they could use the crib for the new baby. The little one loved her big girl bed!! She also weaned the first off the breast at about 18 months in order to be ready for breast feeding the second (and not allowing the first to be jealous or "fight" over the breast). Things worked out great! Different children respond differently and only you know what is best for your daughter. I just thought I would give you an example of what someone else did...you can take it or leave it.
How far along are you?