Massachusetts:
210.25(B) "Exception: Branch ckts supplying lighting outlets in common areas on the same floor as a dwelling unit in a new or existing two-family or an existing three-family building shall be permitted to be supplied from equipment that supplies one or more of those dwelling units."

Development:
Initial consult was over the Phone. Owner 'Implied' Same level both units.
Reality, Same entry, 1 unit up, 1- unit down. So the exception does not apply.

additionally, 1 tenant was paying for he others dryer. A bit of a cluster. (Cha-Ching, If they Keep complaining)

So long story short: A new House panel going in, For common Lights/smokes/area.

Smokes and CO detectors off of the house panel.
common areas interconnected with battery back ups and CO (combos'). Why the EI Is calling this and not the FD...?...
Any way that's it.

Smoke/CO set up:

Photo electric smokes (local requirement), All combo CO(my choice),After the common areas there will be a Heat detector in each unit(inter connected with the common area).
Then off of the heat a 2-wire feed to feed the inter connects in the units. All off of the Hall (1st floor light, they don't like dedicated smoke ckts here, They want you to know there is no power).

This setup ensures that a common area smoke will alert both, A heat in a unit will alert both, And smoke in an individual unit will stay local.

I think I got it covered. (don't mention the heating system shut down on CO detection, Building dept, not involved YET!)