Global models of plasma discharges are a standard tool in plasma fluid simulations incorporating complicated chemistry. These models use global balances of energy and conservation of species number in order to estimate volume averaged number densities and temperatures of plasma components. Simple semi-analytic estimates of species density profiles valid in a wide range of parameters are used in order to include wall fluxes. Due to the nature of the wall fluxes estimation, the global models are limited to single chamber designs for which analytic solutions were obtained. In this paper we present the development of interface bound-ary conditions that allow the use of conventional global models for separate chambers. For this reason we present an approximate solution of an ambipolar diffusion problem in a two-chamber geometry. Additional source terms corresponding to interface bound-ary conditions provide closure for simulations of multi-chamber ion sources. We intend to use the derived boundary conditions in an extension of the Global Enhanced Vibrational Kinetic Model (GEVKM)  with updated hydrogen plasma chemistry  to model multi-chamber negative hydrogen sources. We analyze the derived relations and discuss its applicability for the negative hydrogen ion source developed at IPP Garching.