I'm a software developer and occasionally write user documentation and project proposals for my boss. I've been running into a punctuation problem recently that I've been unable to solve. I have a sentence in the form
SUBORDINATING CLAUSE + "and" + SUBORDINATING CLAUSE, MAIN CLAUSE.
If Jack and Jill were going up a hill and Bob were simultaneously driving down the hill, Bob would run over Jill.
I've been rather confused whether there should be a comma before the and. I realize the two subordinating clauses actually share the if, but a part of me still wants to insert a comma like one would do if the and separated two main clauses. Is there any standard (American) punctuation rule that applies in this sort of corner-case? I realize I could rewrite the sentence in a way to avoid this problem, but I'd really like to know what the generally accepted (American) standard for this is.