Hi Pete,
A lot of the limitations here are due to the underlying technology that NORMA is built on. The Microsoft Modeling Framework (formerly DSL tools) is an in-memory representation of the data with no native integration with external data sources (either other DSL Store objects or external databases). This means that any live coordination between models would be an extensive extension to or the current system or a flat-out rewrite on a different technology stack.
Having said that, there are some 'helps' in the tool that can make this easier. Basically, the slowest loading part of the system is the diagrams, so if you move elements between model files you do not want to take the diagrams. The cross-model drag/drop support also works with groups, so if you create a new group (from the Groups node in the model browser) and add in object types and fact types you want to move to another model you can then drag the group instead of the diagram to the other model.
A subsequent drag of the same group will update the existing items. However, please note that the integration is pattern-based and the internal ids from the original model are not kept on drag. Therefore, if you make a significant change to an object type (change the name, change the id, etc.) or fact type (reading change), the drag may not associate the new object with the old one. Therefore, for this type of change, you would need to manually make the corresponding change in the target model as well.
-Matt
PS I had an external tool years ago that added some automation to this process (including deletion support), but it was never totally stable and would need significant maintenance work to be at a point where I could let it off my machine.