It is shown that the method proposed by Wosiek [Phys. Rev. B 49, 15 023 (1994)] for the calculation of the order-disorder critical temperature, which turns out to be exact when the critical temperature is determined by symmetry properties like self-duality, is, when applied to a system of two coupled Ising planes, the lowest-order approximation in a hierarchy which appears to converge to the exact result.