جَاءَنِي خَالِدٌ. رَحِمَهُ اللَّهُ.
Khalid came to me. May God have mercy on him.
The first is a report; the second, in the intended reading, is a supplication. Keeping them separate protects their different speech functions. With a coordinating wāw in a narrative context, a listener may instead read the second verb as another reported event: God had mercy on him.
Therefore the choice of faṣl cannot be decided merely by thematic relatedness. One must also test whether coordination creates an unintended attribution or interpretation.