Generates ditrans,octacis-undecaprenyl pyrophosphate (UPP) from isopentenyl pyrophosphate (IPP) and farnesyl diphosphate. UPP is the precursor of glycosyl carrier lipid in the biosynthesis of bacterial cell wall polysaccharide components such as peptidoglycan and lipopolysaccharide. (2E,6E)-farnesyl diphosphate + 8 isopentenyl diphosphate = di-trans,octa-cis-undecaprenyl diphosphate + 8 diphosphate Binds 2 magnesium ions per subunit. Homodimer. This enzyme shows a novel protein fold completely different from the 'isoprenoid synthase fold' that is thought to be a common structure for the enzymes relating to isoprenoid biosynthesis. Belongs to the UPP synthase family.