Yeah Iowa I know what you mean by squirrely on the single bike after riding the tandem for a while, it's really noticible in the back of the single bike, I noticed that a lot back when I was first riding tandem, when I got back on the single (or in the tandem world they call them half bikes

) the first 10 or so minutes of the ride, the back just felt weird without that extra person back there...Very weird feeling.
That's cool that you found a used one cause you can use that and work with it to find what the next one will be and the way you want it to fit..
Keep on putting miles on it, and congrats again on the tandem..
Benjamin