Foams age as gas diffuses from smaller bubbles to larger ones. This results in an increase of the average bubble size, which leads to a modification of the foam rheology and overall stability. This is why understanding coarsening is crucial for the creation of aerated materials. The coarsening rate is affected by various mechanisms, which are impacted by geometry (bubble size and bubble size distribution), liquid fraction, air solubility in the liquid phase, permeability of the interfaces, mechanical properties of the liquid phase and of the interfaces. In many real systems changing a single parameter will impact several mechanisms. In this work, we use a two -bubble experiment to isolate and explore the impact of bulk and surface viscosity on the rate of bubble coarsening. The experimental set-up allows us to decouple the impact of viscous dissipation from the rate of gas diffusion. We build a model in which the gas transfer is driven by Laplace pressure differences and hindered by viscous stresses. We need to take into account gravitational stresses, and dissipation in the connections, with which our model is fully predictive of the experimental data.