Some of the discussions about the divergence or convergence of
the two stackexchanges sites Ubuntu and Unix/Linux shows that the discussion is lead not on the same dimension, but on two different ones.
The proponents of a convergence have a scientific categorization in mind, a taxonomy, similar to the way plants and animals are categorized, or the Dewey Decimal system in a library.
For these dimension of thinkers, sites that are subsets of one another, or sites that have considerable overlap in the possible set of questions are an aberration. It does not fit in the universe of this style of thinking.
The other dimension come from the observation of the communities. Communities do not grow according to the taxonomy scientist like to use. Taxonomies are simplifications which might be accurate for the purpose of a limited observation sufficient for the particular science. However, they are not sufficient for the needs and feelings of the communities.