Probably involved in biosynthesis of the precursor for C25 (sesterterpanyl chain) moiety of C20-C25 diether (2-O-sesterterpanyl-3-O-phytanyl-sn-glycer) membrane lipid. Catalyzes the condensation of isopentenyl pyrophosphate with the allylic pyrophosphates to yield geranylfarnesyl diphosphate (GFPP). Geranylgeranyl diphosphate (GGPP) is the preferred substrate, but dimethylallyl diphosphate (DMAPP) and farnesyl diphosphate (FPP) 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.