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it has always been my understanding that stacking chips will not do anything because the first one will always be the one used. or something to that effect.
I read somewhere on another forum that if one is a monitor and one is a programmer you can get an obdII splitter and run both. Not sure though as I haven't done it myself just second hand info.
There is a OBDII splitter and you can have two devices connected to the OBDII port.
But if OBDII diesels work like gassers, the programmer/tuner will over-write the PCM stock program (or whatever program is in the PCM). I'm only guessing on this, but last one in wins (and you may end up with a locked tuner and no start condition).