Fix the split line to the reflected geometry using coincident constraints
- Select
Constraints > 
Coincidence. - Select the split line (A) and the stair or wall line (B).
- Select the split line (C) and the stair line (D).
- Select the split line (E) and the stair or wall line (F).
- Select the split line (G) and the stair or wall line (H).
- Quit by pressing the Esc key.
- Save the project by selecting File > Save.
