Probably involved in biosynthesis of the precursor for C25 (sesterterpanyl chain) moiety of C25-C25 diether (2,3-di-O-sesterterpanyl-sn-glycero) membrane lipid. Catalyzes the condensation of isopentenyl pyrophosphate with the allylic pyrophosphates to yield all-trans geranylfarnesyl diphosphate (GFPP). Geranylgeranyl diphosphate (GGPP) is the preferred substrate, however methylallyl diphosphate (DMAPP), farnesyl diphosphate (FPP) and geranyl diphosphate (GPP) can also be used as allylic substrate. (2E,6E,10E)-geranylgeranyl diphosphate + isopentenyl diphosphate = (2E,6E,10E,14E)-geranylfarnesyl diphosphate + diphosphate Binds 2 Mg(2+) ions per subunit. Homodimer. Belongs to the FPP/GGPP synthase family.