Play in headset with no upper bearings on a tandem bicycle
A single bearing race really isn't very good at resisting torque perpendicular to its axis, so I'd be surprised if there wasn't meant to be a set of bearings in the top. Even closely spaced sets of bearings aren't great in that regard. Unless there's a special arrangement inserted from below, the lack of a top bearing would be likely to cause play, and possibly damage. The lower bearing should be checked in any case. The load on a tandem is quite high so I'd expect a robust design using the full available steerer tube length.