A portable analyser is described for simultaneous detection of CO2 and HF emitted by volcanoes and fumaroles. The system is based on two fibre-coupled distributed feedback lasers and only one multipass cell, and provides the absolute concentration values of the two gases, without requiring a calibration procedure, at a maximum rate of 4Hz. The spectrometer can operate both in a closed-cell configuration and in an open-cell set-up, with the latter mitigating problems associated with chemisorption of the HF molecule. The concept, practical realization and laboratory performance of the device are presented, together with results from a first test campaign measuring volcanic gases emitted from the crater of Vulcano, Italy. We obtained an in-field sensitivity of 320ppb for CO2 and 20ppb for HF at 2's integration time.