Recommendations for search terms - insulate wall between duplex units
There actually aren't any (at least in the Model Codes)

The reason you're coming up empty handed is because the Code makers, by and large, have not addressed this when writing energy conservation codes.  Party (common) walls are generally not considered part of the building thermal envelope, which is actually rather OK, as insulating them poses special challenges.

From the Building America Solutions Center page on the topic (italics in original):


  The model energy codes do not address minimum insulation requirements for common walls because the common wall is not defined as part of the building thermal envelope.


If you want to insulate and/or air seal them, you must respect the wall's fire rating and listing when insulating in order to be Code compliant.  You may wish to speak with your AHJ on this, especially considering that you are dealing with an older style of party wall which behaves differently from modern residential party walls.