Tuning a one row to "just" or "CAJUN" tuning is all about playing in specific keys to make the chords "right"
12TET or standard western tuning is a compromise to make all keys sound "ok".
If you were to tune the 2 row to "just" or "CAJUN" tuning on the G/C
you would have a C row that plays ok to itself and a G row that plays ok to itself but would destroy the purpose of a 5th (4th) apart 2 row system which is "cross-rowing" allowing you to play in a variety of keys that all sound good.
Simply put if you tuned it "just" or "CAJUN" tuning.. it would sound like vaca caca.
Hohners suffer from too much tremolo and absolutely horrible factory tuning.
Tone down the tremolo a bit and have it properly tuned.
Bob at The Button Box is my choice of tuners.. there are others.. ( some to be avoided)
And do not get talked into "DEDIC" tuning as is a favorite of one guy back east. He has ruined a few Castagnaris ( my opinion) Castagnaris were not built and designed for DEDIC tuning.
Ask B LaFleur if he wants to take a stab at making a Hohner 2 row sound better.
Excellent... he can guide you through the technical reasons.. but what it boils down to is the way you would tune a note on the G row would not be the same as that note on the C row.. ergo C L A S H !!!!