The eruptive activity at the neighbouring Hawaiian volcanoes, Kīlauea and Mauna Loa, is thought to be linked despite both having separate lithospheric magmatic plumbing systems. Over the past century, activity at the two volcanoes has been anti-correlated, which could reflect a competition for the same magma supply. Yet, during the past decade Kīlauea and Mauna Loa have inflated simultaneously. Linked activity between adjacent volcanoes in general remains controversial. Here we present a numerical model for the dynamical interaction between Kīlauea and Mauna Loa, where both volcanoes are coupled by pore-pressure diffusion, occurring within a common, asthenospheric magma supply system. The model is constrained by measurements of gas emission rates indicative of eruptive activity, and it is calibrated to match geodetic measurements of surface deformation at both volcanoes, inferred to reflect changes in shallow magma storage. Although an increase in the asthenospheric magma supply can cause simultaneous inflation of Kīlauea and Mauna Loa, we find that eruptive activity at one volcano may inhibit eruptions of the adjacent volcano, if there is no concurrent increase in magma supply. We conclude that dynamic stress transfer by asthenospheric pore pressure is a viable mechanism for volcano coupling at Hawai‘i, and perhaps for adjacent volcanoes elsewhere.