Dry wall access between the two units of a duplex

I want to provide access through the common wall of the two units of a duplex. nothing fancy, sort of a "doorway" without a door, same size of a door. Possible a dry wall access, could be cased, whatever requires minimum work. I do not want to re-zone the house to sfh, at some point if I want to close the access I want to be able to. 

I learned that separation wall for a duplex could be a fire wall. Possibly could be a load bearing wall, not sure.  I am sending contractors to give me an idea of what it would take, but wondering if someone on bg has already done it and has advice in this regard. Is it feasible....if the structural integrity of the house is maintained, are there other issues to consider. Thanks!