We know even less about the second act of the play The King In Yellow than we do of the first. It is not stated whether there are any further acts following the second, but whilst this is possible, what little we are told indicates that there may only be the two. Following the example of the First Act, it is possible there are only two scenes, but we know nothing of them.
In The Repairer of Reputations, we are told that Hildred Castaigne, having read the first act, threw the play into the fire, but, catching sight of the opening words of the second act decided to rescue it. We are not told what these words were or their importance to Castaigne, but it seems likely he felt they supported his royal claim in some way.
It is possible but unlikely that the bitter cry of Cassilda could come from the Second Act rather than the First.
It is stated that it is the Second Act that is the real source of horror in the play, and that the banality and innocence of the First Act belies the awful effect of the rest (the use of the indefinite term 'rest' here allows the slight possibility of further acts after the second).
We know nothing of the plot, setting or characters of the Second Act, nor how they relate to those of the First Act.