There is an even simpler solution to merging building geometries:

Open Sketchup twice:

• First for your OSM-file No1
• Second for your OSM file No 2

Now apply COPY to the desired OpenStudio object (space, zone) in Sketchup#1. Then apply PASTE in PLACE in Sketchup#2.

Now you have the desired additional space in your model (in Sketchup#2) and you only need to take care of adjacent surfaces (intersections, surface matching).

In the example displayed it only takes a simple move command (m) to get the spaces adjacent and to adjust the window at the west side wall.

FINISHED !

(In this example there were no duplicate names)