  We propose a simple model of two coupled mesoscopic rings threaded by
magnetic flux which mimics a device for electron transmission in a controlled
fashion. Within a tight binding formalism we work out exactly the conditions
when a completely ballistic transmission can be triggered at any desired
position of the Fermi level within a continuous range of energy. The switching
action can be easily controlled by appropriately tuning the values of the flux
through the two rings. The general transmission across the ring system displays
interesting features characterized by the appearance and collapse of Fano
resonances, and the suppression of the Aharonov-Bohm oscillations in several
cases. We report and analyze exact analytical results for a few such cases.
