Catalyzes the condensation of isopentenyl pyrophosphate (IPP) with geranyl diphosphate (GPP) to yield (2E,6E)-farnesyl diphosphate (E,E-FPP). May be used for squalene and possibly sterol biosynthesis. (2E)-geranyl diphosphate + isopentenyl diphosphate = (2E,6E)-farnesyl diphosphate + diphosphate Binds 2 Mg(2+) ions per subunit. Isoprenoid biosynthesis; farnesyl diphosphate biosynthesis; farnesyl diphosphate from geranyl diphosphate and isopentenyl diphosphate. Belongs to the FPP/GGPP synthase family.