Why can't I charge my smartphone wirelessly and by cable at the same time or charge two devices at the same time?

Smartphones are designed for only one or other of these charging modes to work at a given time.

On account of the maximum charging power available, it isn't possible to charge two devices at the same time via the cradle, for example to charge the smartphone wirelessly and another device by cable. The charging power would not be enough to charge two devices simultaneously.