Converts farnesyl diphosphate (FPP) into squalene, a precursor for sterol biosynthesis in eukaryotes. 2 (2E,6E)-farnesyl diphosphate + H(+) + NADH = 2 diphosphate + NAD(+) + squalene 2 (2E,6E)-farnesyl diphosphate + H(+) + NADPH = 2 diphosphate + NADP(+) + squalene Belongs to the phytoene/squalene synthase family.