I've only ever seen two of these and they were made by the same person from Massachusetts (USA). However, from what I recall, I want to say that they may have been made by a sail maker (e.g., North Sails) using a pattern / design provided by the tandem owner and it wasn't overly complex...
The primary purpose was to prevent other things in their mini-van from rubbing up against the greasy drive train or paint finish.
You might want to shoot an Email to the company at this link and see what products / leads they have to offer as they seem to carry several different types of single bike storage bags. They would most likely know if their suppliers also offer larger versions for tandems: http://www.cdisports.com/index.php?main_page=contact_us
Regardless, if you don't find a lead on a commercially available bag, you might want to call a sail maker or some other business like an auto upholstery shop or canvas tent / canopy maker to see if they would be able to make you an oversized storage bag out of some ballistic nylon or other lightweight but tear-resistant and breathable fabric. I would think that you could simply build a pattern for your bike by laying it out on butcher paper for the profile and then add the needed girth dimensions for the width + 10% or so all around and have a fairly custom bag.
Anyway, just my .02 as that's what I would likely do if I wanted something like that.