Catalyzes the sequential condensations of isopentenyl pyrophosphate (IPP) with dimethylallyl diphosphate (DMAPP) to yield geranyl diphosphate (GPP) and with GPP to yield (2E,6E)-farnesyl diphosphate (E,E-FPP). dimethylallyl diphosphate + isopentenyl diphosphate = (2E)-geranyl diphosphate + diphosphate (2E)-geranyl diphosphate + isopentenyl diphosphate = (2E,6E)-farnesyl diphosphate + diphosphate Binds 2 Mg(2+) ions per subunit. Isoprenoid biosynthesis; geranyl diphosphate biosynthesis; geranyl diphosphate from dimethylallyl diphosphate and isopentenyl diphosphate: step 1/1. Isoprenoid biosynthesis; farnesyl diphosphate biosynthesis; farnesyl diphosphate from geranyl diphosphate and isopentenyl diphosphate. Contains two aspartate-rich DDxxD motifs, designated as FARM (the first aspartate-rich motif) and SARM (the second aspartate-rich motif). Belongs to the FPP/GGPP synthase family.