In this paper we discuss how the standard optimal Wiener filter theory can be applied, within a linear approximation, to the detection of an isotropic stochastic gravitational-wave background with two or more detectors. We apply then the method to the AURIGA-NAUTILUS pair of ultra low temperature bar detectors, near to operate in coincidence in Italy, obtaining an estimate for the sensitivity to the background spectral density of ? 10-49 Hz-1, that converts to an energy density per unit logarithmic frequency of ? 8 10-5 rhoc with rhoc ? 1.9 10-26 kg/m3 the closure density of the Universe. We also show that by adding the VIRGO interferometric detector under construction in Italy to the array, and by properly re-orienting the detectors, one can reach a sensitivity of ? 6 10-5 rhoc. We then calculate that the pair formed by VIRGO and one large mass spherical detector properly located in one of the nearby available sites in Italy can reah a sensitivity of ? 2 10-5 rhoc while a pair of such spherical detectors at the same sites of AURIGA and NAUTILUS can achieve sensitivities of ? 2 10-6 rhoc.
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