Converts the C20 geranylgeranyl diphosphate (GGPP) to the C40 lycopaoctaene, the first committed intermediate in the production of lycopadiene (PubMed:27050299, PubMed:28813599). Converts farnesyl diphosphate (FPP) into squalene, a precursor for sterol biosynthesis in eukaryotes (PubMed:27050299, PubMed:28813599). Converts with low efficiency the C20 phytyl diphosphate (PPP) to the C40 lycopadiene in vitro. This reaction may not have biological significance in vivo (PubMed:27050299). 2 (2E,6E)-farnesyl diphosphate + H(+) + NADH = 2 diphosphate + NAD(+) + squalene 2 (2E,6E)-farnesyl diphosphate + H(+) + NADPH = 2 diphosphate + NADP(+) + squalene 2 (2E,6E,10E)-geranylgeranyl diphosphate + H(+) + NADPH = all-trans-lycopaoctaene + 2 diphosphate + NADP(+) Belongs to the phytoene/squalene synthase family.