If you placed more than one device on the charging pad, the device with the weakest battery would most likely try to pull more power than the other nearby devices. There is a limited number of magnetic flux lines. The magnetic field flux lines would concentrate on the device pulling the most power, leaving fewer lines for the rest.

Think of the multiple devices near the main coil as separate windings on a transformer. If the transformer is rated at say 5 watts, you will not be able to pull more than that from any one winding. If each winding has roughly the same load on it, the total should not exceed 5 watts.