"I still haven't signed up for BJs yet... it will likely be BJs, since Costco requires proof of second cardholder living at the same address....."

You should sign up for whatever place you want which is closer to you/has the stuff you want. (I'll either never use that dupe card or if I do it will be only once a year or something - we don't use any food or goods from there, so it would only be for the off chance of bulk-purchase of Polaroid film for trooping or something). If there is someone else who can better use it, you should definitely pass it to them :)