The presence of a jāmiʿ supports the possibility of conjunction, but the clauses must also express distinct intended judgments. If the second only substitutes for the first, the relation may be extremely close while faṣl remains appropriate.
Similarly, wāw can create an unintended scope under a preceding quotation, negation, command or condition. We must distinguish grammatical or rhetorical permissibility from the further elegance of a particular coordinated form.