once i have two new septic systems i'd like to build an addition which would have a 3bdrm apt in it--essentially i'd have a duplex of sorts in the long run.

i'm wondering how best to approach the town about this.

the home directly north of me has several apts in two buildings, (possibly not legal) as does the site two doors south, which has three mobile homes sharing a legal ten bdrm capacity septic.
any tips on how to help get this approved?

Here, what you are proposing would never be allowed on any normal "homeowner" or "estate" sized lot. You would have to have quite a lot of acreage, and would be required to split it into separate lots, even if you retained ownership of all. A single "normal" sized property would have to be zoned for multi family dwellings to even consider it. And the health department would not allow two active septic systems on one property, even if they were serving one larger single building raather than multi family structures. A new septic system would have to be designed for the larger home, not adding a different system to serve the additional bedrooms.

There will be a lot of very specific hoops to go through for what you want to occur legally. It might get pretty ugly if your neighbors have done their remodeling without municipal or health department approval. Bringing that to the authority's attention as a rationale for them approving your project would likely backfire. You've got your work cut out for you, and the first step is to stop at your local codes office.