We investigate the first law of thermodynamics in the stationary axisymmetric configurations composed of two Kerr black holes separated by a massless strut. Our analysis employs the recent results obtained for the extended double-Kerr solution and for thermodynamics of the static single and binary black holes. We show that, similar to the electrostatic case, in the stationary binary systems the thermodynamic length $ell$ is defined by the formula $ell=Lexp(gamma_0)$, where $L$ is the coordinate length of the strut, and $gamma_0$ is the value of the metric function $gamma$ on the strut.
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