Mauna Loa and Kilauea volcanoes, Hawaii, are thought to be coupled by pore pressure diffusion through an asthenospheric melt layer. However, abundant observations of independent activity of these volcanoes suggest a more complicated relationship. Here we analyze surface deformation data, deep seismicity and gas measurements, to reveal strong coupling of these volcanoes between 2003 and 2008. In early 2005, we find a shift from anticorrelation to correlation of magma-chamber inflation. The shift is preceded by a seismic swarm in the mantle beneath Mauna Loa and accompanied by a large silent slip event beneath the south flank of Kilauea. This suggests that these volcanoes are coupled during mantle-driven surges and that the 2005 silent slip event was triggered by accelerated magma supply at Kilauea.