Ok, I gave them more than a couple weeks to fix the error, and not having received a response, sent them another email asking in effect, ***?
In response, they said I'd have to submit a second time, using a different receipt. It's obvious that this rep didn't read the email history, and in this case did not understand my question about being shortchanged with only one rebate reward on a single purchase of two 5-qt jugs.
At this point, I'm not holding out much hope they'll understand their error, never mind fix it, so perhaps I'll just write off the $5 from "winning" the game and call it even. Not going to put any more effort into it.