The present invention is directed to golf balls comprising a fluid-filled center surrounded by an outer core, wherein the outer core comprises a layer formed from a low modulus HNP composition and a layer formed from a high modulus HNP composition. The present invention is not limited by which outer core layer is formed from the low modulus HNP composition and which layer is formed from the high modulus HNP composition, so long as both layers are present in the outer core of the golf ball. Low modulus HNP compositions of the present invention comprise a highly neutralized acid copolymer having a modulus of from 1,000 psi to 50,000 psi. High modulus HNP compositions of the present invention comprise a highly neutralized acid copolymer having a modulus of from 25,000 psi to 150,000 psi.