You can combine models that do not have a common ancestor by merging
their structures. When you structurally merge two models, you designate a
source and target model, and then merge the content of the source model into
the target model. Structural merging is different than merging models that
have a common ancestor. When you merge models that have a common ancestor,
you compare their model element identifiers.
You can attempt to structurally merge two models automatically by adding
new objects without deleting or changing existing ones. If the merge cannot
automatically resolve all differences and conflicts, then you must manually
resolve them.