A theoretical basis for the application of a "two-wire thermocouple" comprised of two fine thermocouples of different diameters to fluctuating temperature measurements is established with the aid of the least squares method. Based on this theory, the simultaneous in situ reconstruction of thermocouple time constants and compensated temperatures is realized without using any geometrical features of the two thermocouples such as wire diameters. Thus, introduction of a 'time-constant ratio' into the reconstruction scheme is unnecessary. Consequently, problems in two-wire thermocouple techniques proposed so far can be overcome. In addition, a simple and reliable method for correcting heat loss due to thermal radiation from the thermocouples is devised, which can be incorporated into the reconstruction scheme with little modification. The developed scheme is tested in a fluctuating temperature field with a diffusion flame formed in a two-dimensional wind tunnel, It is shown that the present technique is successful and allows us to investigate the characteristics of time-constant fluctuations in high-temperature turbulent flows.