Floating offshore wind and wave energy are both promising marine renewable energy technologies that are still at an early stage of development. This paper reviews synergies between the two technologies that can be mutually beneficial and facilitate the path to commercial exploitation. The analysis focuses on three different types of combined wave and wind systems: Co-located arrays, hybrid systems and island systems. The decarbonisation potential of co-located systems is discussed. Existing carbon emissions from marine technologies are compared and how synergies can achieve these reductions are explored. A case study is presented to calculate the impact of multi-purpose platforms on the Portuguese energy system. Portugal was chosen because it has an excellent climate for marine resources, deep waters, positive political support for renewable marine energy and previous success in deploying first technology demonstrators for floating wind and wave energy systems.