How to map a place with two units?

Most frequently the two units are mapped as nodes inside the building. The address often includes a unit suffix such as A / B ..., or 100 / 200 .... which is recorded in addr:unit.

If the units are essentially separate buildings with separate entrances, exits and services then you can split the building as is done for e.g. terraced houses.

For cases where the different units are just internal divisions of a large building then the simple indoor tagging scheme can be used. (There are some simple indoor tagging presets for JOSM to same on typing)

If there are more significant structural divisions between part of the building then building:part=* can be used on the individual parts.

For both the indoor tagging scheme and the building parts method the outline of the building should be retained as a building=retail way or multipolygon.

The address scheme has tags for unit number if this is displayed somewhere.

In most cases getting reasonably accurate internal dimensions is very difficult and nodes are used instead. If this approach is used it can still be useful to map entrance nodes on the perimeter.