Catalyzes the synthesis of curcumin by condensing feruloyl-CoA with a diketide-CoA in the curcuminoid biosynthesis. (E)-feruloyl-CoA + (E)-feruloylacetyl-CoA + H2O = CO2 + 2 CoA + curcumin kcat is 0.41 min(-1) with feruloyl-CoA. kcat is 0.94 min(-1) with p-coumaroyl-CoA. Optimum pH is 8.0. Optimum temperature is 45-55 degrees Celsius. Secondary metabolite biosynthesis; flavonoid biosynthesis. Homodimer. Belongs to the thiolase-like superfamily. Chalcone/stilbene synthases family.