You can generally use an access easement for egress purposes. However, there are a bunch of specific requirements to make it work. Check with your local building department about what they require.

Using a common stair, even if allowed under the building code via an easement, might still not be allowed under the zoning code, because it would technically make the dwellings attached, not detached. You might not be allowed to have any built structure in the access easement in order to make it work.

Also review the fire rating requirements for exterior egress courts and confirm that if the common stair is used for egress from the adjacent properties that you still have enough width for those occupants as well... The former may not be an issue if you are R-3.

Where are you?  Egress in the way you describe is usually possible as long as the easement leads to a public way, like a street.
 

Sounds like you want to create a 4 unit building, then divide it along the property line w a fire wall? And put a common stair in the prop line too?

Main issue is setback. It's a zoning issue. Go discuss your scheme w zoning, planning and building depts.

You are probably looking at a variance. Why not ask for a variance to join the lots and do a 5 or 6 unit?

How much space between units?

I'm located just outside of Boston, MA.

Yes, the intent is to create a multifamily where only 2-families are allowed, and this

is because of the desire to save sf on staircases and/ or entries & halls.  It would essentially be a 4-unit building that is divided by a fire wall.

Building code could be satisfied easily I now see, but my worry is that zoning will brand this as a 4 unit, not (2) 2-units.  The norm in the neighborhood is attached brick rowhouses, so I'm not concerned about getting a side yard variance.

Just trying to get as many good insights as possible before going down to zoning and having an assistant planner, two years out of school, tell me what is or is not possible.

"2 years out of school"

Such views of building review staff explain why you seek free advice rather than spend a few dollars and hire an architect. I bet you also go to the hospital or mechanic and tell them what to do as well following free online forums.

OP, I work in the Boston area. I prefer open conversations w building and planning depts. easier and cheaper than having to backtrack. Contact me if you need a local and affordable licensed arch.