Q. Would you use a comma between two independent clauses that are both subordinate to the same “if”? For example, “The qualifying relative would have suffered if the waiver had been denied[,] and the applicant had not been present to help her through her cancer treatments.” I’m looking in CMOS 6.26, but the advice there doesn’t exactly match this situation. Could you please assist?
A. Good question! Though retaining the comma in an example like yours wouldn’t be strictly wrong, omitting it will help readers understand that both clauses following the subordinating conjunction if (“the waiver . . .” and “the applicant . . .”) depend on that same word. This logic would continue to hold if you were to invert the sentence (though a comma would then be required after the second if-dependent clause): “If the waiver had been denied and the applicant had not been present to help her through her cancer treatments, the qualifying relative would have suffered.”
See also CMOS 6.24 (commas with introductory dependent clauses) and 6.25 (commas with dependent clauses following the main clause).