Catalysis of the reaction: 2 farnesyl diphosphate = diphosphate + presqualene diphosphate.
Source:EC:2.5.1.21
Comment
Note that the two reactions performed by EC:2.5.1.21 are represented in GO by 'farnesyl-diphosphate farnesyltransferase activity ; GO:0004310' and 'squalene synthase activity ; GO:0051996'.