  We study the equivalence of ensembles for stationary measures of interacting
particle systems with two conserved quantities and unbounded local state space.
The main motivation is a condensation transition in the zero-range process
which has recently attracted attention. Establishing the equivalence of
ensembles via convergence in specific relative entropy, we derive the phase
diagram for the condensation transition, which can be understood in terms of
the domain of grand-canonical measures. Of particular interest, also from a
mathematical point of view, are the convergence properties of the Gibbs free
energy on the boundary of that domain, involving large deviations and
multivariate local limit theorems of subexponential distributions.
