Converts farnesyl diphosphate (FPP) into squalene, a precursor for sterol biosynthesis in eukaryotes. Does not possess botryococcene synthase activity. 2 (2E,6E)-farnesyl diphosphate + H(+) + NADPH = 2 diphosphate + NADP(+) + squalene 2 (2E,6E)-farnesyl diphosphate + H(+) + NADH = 2 diphosphate + NAD(+) + squalene Transient induction with an apparent maximum occurring 6 to 8 days after subculturing. Belongs to the phytoene/squalene synthase family.