Hi Eric I just looked at some houses in the Portland, OR area, built in the early 1900's, that have been converted to multiplex's. The main issue is whether the house was originally built as a duplex or was converted to one.
Typically, the converted single family houses have non-weight bearing walls installed as simple partitians that can be removed. Doorways have to be either covered over or restored and add-on kitchens removed and plumbing capped before refinishing walls.
However, if the house was designed as a duplex, this could get much more complicated. Dividing walls are often weight bearing, meaning doorways or large weight bearing beams will have to be installed. Even if opened up, the now single house will often have an odd floor plan that will require careful design and layout work to keep it from looking kind of funky.