Simultaneous fluorometric sensing of two analytes becomes possible using a modified dual lifetime referencing (m-DLR) method. In this scheme, two luminescent indicators are needed that have overlapping absorption and emission spectra but largely different decay times. They are excited by a single light source, and both emissions are measured simultaneously. In the frequency domain m-DLR method, the phase of the short-lived fluorescence of a first indicator is referenced against that of the long-lived luminescence of the second indicator. The analytical information is obtained by measurement of the phase shifts at two modulation frequencies. The method is demonstrated to work for the case of dually sensing oxygen and carbon dioxide. It benefits from simple instrumentation and optical setup. The approach is perceived to be of wide applicability. Examples include (1) analysis of two luminescent analytes, (2) analytical determinations that make use of two probes, and (3) sensing of two species such as carbon dioxide and oxygen (as demonstrated here), or oxygen and chlorophyll, provided the luminophores meet the condition of having largely different decay times and overlapping absorption and emission spectra.