Catalysis of the reaction: 2 3-acyl-sn-glycero-1-phospho-(1'-sn-glycerol) = 3-acyl-sn-glycero-1-phospho-(3'-acyl-1'-sn-glycerol) + sn-glycero-1-phospho-(1'-sn-glycerol).
bis(monoacylglycero)phosphate synthase activity