With a single-POP ECU, you need to add the values of the 2 MAFs, since the ECU expects the MAF to read the total volume of air entering. With a dual-POP ECU, you need to average the values of the 2 MAFs, since the ECU assumes that the MAF only reads half the total air volume.
Charles Carbon Black '95 TT Carbon Black '03 BMW M5 '02 Honda CR-V dogmobile