Bounds of impedance matrix of a passive two port network
A necessary condition for an impedance matrix to be passive is that its Hermitian part must be nonnegative-definite along the imaginary axis (of Laplace transform domain). See this paper for detail. The Hermitian part is defined as \$Z_H(s) = (Z(s) + Z^H(s))/2\$. For reciprocal networks, this corresponds to the real part of the impedance matrix. Next, for a 2-by-2 real symmetric matrix to be positive semidefinite, a necessary condition is that its determinant be greater than zero. This then gives the condition you asked. A simple way to appreciate the presence of the Hermitian part has been posted here.