  We analyze a two qubit parity measurement based on dispersive read-out in
circuit quantum electrodynamics. The back-action on the qubits has two
qualitatively different contributions. One is an unavoidable dephasing in one
of the parity subspaces, arising during the transient time of switching on the
measurement. The other part is a stochastic rotation of the phase in the same
subspace, which persists during the whole measurement. The latter can be
determined from the full measurement record, using the method of state
estimation. Our main result is that the outcome of this phase determination
process is {\em independent} of the initial state in the state estimation
procedure. The procedure can thus be used in a measurement situation, where the
initial state is unknown. We discuss how this feed-back method can be used to
achieve a high fidelity parity measurement for realistic values of the
cavity-qubit coupling strength. Finally, we discuss the robustness of the
feed-back procedure towards errors in the measurement record.
