Catalyzes consecutive E-type condensation of two isopentenyl pyrophosphate (IPP) molecules with an allylic substrate such as geranylgeranyl diphosphate (GGPP), farnesyl diphosphate (FPP) or geranyl diphosphate (GPP) to yield the medium-chain product trans-C30-hexaprenyl pyrophosphate (HexPP). GGPP is the physiological substrate. (2E,6E,10E)-geranylgeranyl diphosphate + 2 isopentenyl diphosphate = all-trans-hexaprenyl diphosphate + 2 diphosphate Binds 2 Mg(2+) ions per subunit. Optimum pH is 6. Highly thermostable, remains fully active after 2 hours at 80 degrees Celsius. Homodimer. Belongs to the FPP/GGPP synthase family.