We present a setup of two coupled micromaser cavities for studying decoherence. The system is prepared in a state where, initially, one photon is contained in one of the two cavities. The coupling of the two resonators allows the photon to tunnel into the other cavity with a certain probability. The system thus oscillates between the two states parallel to 1 >(L)parallel to 0 >(R) and parallel to 0 >(L)parallel to 1 >(R). It is shown how to map these states onto the states parallel to g > and parallel to e > of probe atoms used to investigate the status of the cavity system. It is an important result that the symmetry of our special setup allows decoherence to be monitored by measuring just the diagonal elements of the atomic density matrix. This property relies on the fact that only parallel to 0 > and parallel to 1 > states are relevant. To our knowledge, this is the first proposal that tries to exploit this particular property.