We study whether the stationary state of two bulk-driven systems slowly exchanging particles can be described by the equality of suitably defined nonequilibrium chemical potentials. Our main result is that in a weak contact limit, chemical potentials can be defined when the dynamics of particle exchange takes a factorized form with respect to the two systems, and satisfies a macroscopic detailed balance property at large deviation level. The chemical potentials of systems in contact generically differ from the nonequilibrium chemical potentials of isolated systems, and do not satisfy an equation of state. Yet, classes of systems satisfying the zeroth law of thermodynamics can be defined in a natural way. These results are illustrated on a driven lattice particle model and on an active particle model. The case in which a chemical potential cannot be defined also has interesting consequences, like a non-standard form of the grand-canonical ensemble.