Tandems often use hubs with regular threading on both sides. right hand threads on the right side for a standard freewheel, right hand threads on the left side for a threaded disk (old school Shimano) or drum brake (Arai) where you don't need a lock ring because the braking action just tightens up the disk more. Tandem riders don't worry about spinning the disk off when going in reverse. Many single cog freewheels can be threaded on backwards so that would work on the left, backwards so the pawls face the correct direction. I'd use some medium threadlocker. Running a double chain means the left won't be taking all the pedaling force so it's not likely to spin off. FYI, when you put a single freewheel on backwards, the slots for the removing tool will be inaccessible. And then there are the single cog freewheels that never had slots for tools.