Catalyzes the synthesis of Z,E-mixed prenyl diphosphates by a condensation of isopentenyl diphosphate to an allylic diphosphate. It shows a large substrate specificity accepting dimethylallyl diphosphate (DMAPP), GPP, E,Efarnesyl diphosphate (FPP), E,E,E-geranylgeranyl diphosphate (GGPP), neryl diphosphate (Z-GPP), and (2Z,6E)-farnesyl diphosphate (Z,E-FPP) as allylic substrates. The enzyme exhibits the highest activity when Z,E-FPP is employed as an allylic substrate. The major product is dodecaprenyl diphosphate (C60) under every allylic substrate conditions, but the enzyme is also able to synthesize even C70 prenyl diphosphate as the maximum chain-length product. (2Z,6E)-farnesyl diphosphate + 10 isopentenyl diphosphate = di-trans,deca-cis-tridecaprenyl diphosphate + 10 diphosphate (2Z,6E)-farnesyl diphosphate + 11 isopentenyl diphosphate = di-trans,undeca-cis-tetradecaprenyl diphosphate + 11 diphosphate (2Z,6E)-farnesyl diphosphate + 9 isopentenyl diphosphate = di-trans,nona-cis-dodecaprenyl diphosphate + 9 diphosphate Binds 2 magnesium ions per subunit. Homodimer. Belongs to the UPP synthase family.