Is the little magnet that secures to the spokes, should it be affixed to both spokes near where they cross close to the hub?? Or is the unit meant to be attached to only one spoke??

Ideally closer to the hub where the rotation velocity is slower, and where the fork sensor can be closest .

One spoke. there is a notch in one end of the plastic piece, which should fit the spoke, and then the magnet screws down on that to keep the magnet in place.

It seems to work OK, and I don't have a particular problem with it, but you do need to check the magnet hasn't been knocked out of alignment when fixing a flat, or if you need to remove the wheel for transport.