The textbook uses a historical conditional divorce formulation with entry into two houses. The particle fa connects entry into the second house to the immediately preceding act.
«إن دخلت هذه الدار، فهذه، فأنت طالق»
If you enter this house and then this other house, you are divorced.
On this formulation, the required sequence is entry into the first house followed by entry into the second without a separating interval. Merely entering both houses at unrelated times does not satisfy the same textual relation.
This is an illustrative juristic hypothetical about wording, not advice on actual divorce cases.