Catalyzes the sequential condensation of isopentenyl diphosphate (IPP) with (2E,6E)-farnesyl diphosphate (E,E-FPP) to yield (2Z,6Z,10Z,14Z,18Z,22Z,26Z,30Z,34E,38E)-undecaprenyl diphosphate (di-trans,octa-cis-UPP). 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. Belongs to the UPP synthase family.