I was very frustrated when I merged two models the other day. The background is that the requirement for the building I am designing changed. It has a strange custom shape, a bit like a water tower that looks like a fuel filter (an inverted Pyramid).
When changing the elevation of the levels, the masses and curtain walls that were created on them failed spectacularly and most of the model was deleted.
Instead, I followed a different approach. Rather split the model in two (base and top). Since the base had to be lifted, I linked the Base model into an empty file and bound the link. It did not cause too much of a problem.
The top model was also linked in. Initially, I made the mistake of binding the elements into levels in the model itself, and when one of the levels was not aligned, the proves went haywire, and I ended up losing more time.
When you do the same as me (recompose a model to change the elevations of levels that refuse to be moved without consequence), then rather link in the Levels when you bring the model in as well. That way you are guaranteed success.

Merging models like this is not recommended practice. Breaking them up and linking them into each other is recommended, especially to keep the model sizes down, but reconstituting them is frowned upon, for good reason.
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