First, you got it backwards: denormalization joins relations, normalization splits them;
Second, if they share the same key, then it's not normalization, as the the split relations have distinct keys.
Second, if they share the same key, then it's not normalization, as the the split relations have distinct keys.
Normalization is the process of organizing the attributes and relations to, among other things, eliminate redundancy. 2 relations that store and share the same primary key is by definition redundant.
Splitting relations for the sake of skinnier tables is creating redundancy(the primary key is stored twice) and insert/update anomalies and can be reasonably thought of as a denormalizing behavior.