If you went with 2 freewheels, you wouldn't really NEED lockrings...
but there might be a slack point because of the way a free wheel works. I think it would cooler with two fixed gear cogs and forget the freewheel al together. Maybe try and find a steel flip-flop hub and tack weld the cog onto the side that doesn't have the lock ring threads? Sorry, I don't know a real lot about flip-flop hubs. Good plan, it's going to take some expirimentation.