Thanks for the question. That's trickier than it looks because you need the alignment to be different and gradually change from the beginning of the flight to the end. In reality the lines that drive two flights of stairs do not connect neatly at the corners like that. You can see this if you look at a flight of stairs with strings. One is above the other on the corner.
So on option would be to break the spline on the corners and move up the bottom vertex.
Another possible solution would be to add a Z offset to the bottom of the stairs that gradually reduces for each additional step. To do this you can use the expression (1-XSectionPosition)*Input1
XSection Position returns the current position of the step on the spline section as a scalar value between 0.0 at the start to 1.0 at the end. You can use this to multiply an offset value that will gradually reduce along the spline.
Please take a look at the attached sample file for an example of this.
I hope that helps, please let me know if you have any further questions.