Catalyzes the condensation of 2 two farnesyl pyrophosphate moieties to form squalene. It is the first committed enzyme of the sterol biosynthesis pathway. Required for the biosynthesis of ergosterol (By similarity). 2 (2E,6E)-farnesyl diphosphate + H(+) + NADPH = 2 diphosphate + NADP(+) + squalene 2 (2E,6E)-farnesyl diphosphate + H(+) + NADH = 2 diphosphate + NAD(+) + squalene Terpene metabolism; lanosterol biosynthesis; lanosterol from farnesyl diphosphate: step 1/3. Monomer. Belongs to the phytoene/squalene synthase family.