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The characterisation of dual-output Mach-Zehnder interferometers operating with long-range surface plasmon-polaritons at a free-space wavelength of ~1370 nm is reported. The devices were constructed by embedding Au stripes in Cytop claddings, and consist of a symmetric Mach-Zehnder interferometer in cascade with a 50:50 coupler. By injecting electric current via probes to generate heat in the active region in one arm of the interferometer, a phase difference between the arms was thermo-optically induced, modulating the optical power of the two outputs. The outputs were complementary as expected theoretically, thus demonstrating the switching abilities of the structure. The advantages for sensing applications of a dual-output interferometer over a single-output one are a 2X larger dynamic range and the ability to cancel common noise and source fluctuations. The larger dynamic range and noise cancellation produced a minimum detectable phase shift 4X lower than obtained by monitoring a single output. The smallest value of Δφmin obtained was ~3 mrad. The structure is especially promising for (bio)chemical sensing applications.