I have a sentence with a rather complex 'so that' subordinate clause that contains two clauses, each of which could stand alone as a complete sentence. I know the comma before 'so that' would be wrong, but should I use a comma between the two clauses within the 'so that' phrase? For example:
"For the experiment, the restriction enzymes were digested so that the inserts became larger and it was easier to see the final product."
"For the experiment, the restriction enzymes were digested so that the inserts became larger, and it was easier to see the final product."
The original sentence was far more complex than this; each clause had additional elements and/or parenthetical information, stretching the entire thing out over four lines of text. Arguably a comma would make the clauses easier to digest, kinda like the restriction enzymes, but I'm not so sure how strict the rule on omitting commas before essential subordinate clauses is and whether it stretches to additional clauses within.