Connect 1st PLX to AD1+ signal and AD1- ground.
Connect 2nd PLX to AD2+ signal and AD2- ground.
Log the pids EXT.AD1 and EXT.AD2.
Select the calc pids CALC.AFR_PLX1/2 and CALC.BEN_PLX1/2.
In FVS2 device settings, checkmark the box to always include External Analog Pids.