  A macroscopically constrained Wang-Landau Monte Carlo method was recently
proposed to calculate the joint density of states (DOS) for systems with
multiple order parameters. Here we demonstrate results for a nearest-neighbor
Ising antiferromagnet with ferromagnetic long-range interactions (a model
spin-crossover material). Its two relevant order parameters are the
magnetization $M$ and the staggered magnetization $M_{\rm s}$. The joint DOS,
$g(E,M,M_{\rm s})$ where $E$ is the total system energy, is calculated for zero
external field and long-range interaction strength, and then obtained for
arbitrary values of these two field-like model parameters by a simple
transformation of $E$. Illustrations are shown for several parameter sets.
