Hiya, you mean the Mirror Room? I thought at first that it would be the same process as for the Galleries of Me (the latter which I luckily could experience once during my gameplays), but you don't need to switch between reality and dream to get that one.
Upon looking through some external source (most probably the Uboachan threads), the correct solution was revealed... You just need to enter and exit the balcony of Urotsuki's room (dream version) and this in a repeated way.
I even tested it in a recent version of Yume 2kki and it worked.
Good luck! Have fun!
(By the way, the first time I got it -by accident-, I had unequipped all of the effects to get the endings... No fairy or child form to access the old train station part... How bummed I was)