You should consider different topics:
- Master-Data merge
- Extract-Transform-Load (ETL) processes from one org to the other
- Business process integration
- Integration of external systems
- User training
In general we can assume that the data model differs. Therefore the existing data models have to be compared and merged into a single model.
List all existing fields and objects (add a description) and create a mapping between the data models.
The extraction, transformation and loading of data from one system into the other is another topic to consider. You should make sure that the users can work with the “new” and merged system without shutting the system down for several days. Therefore you should plan the data migration upfront. It is crucial to test the migration.
Import the data into a full sandbox, use a tool which allows you to reproduce and automate the migration.
Business process integration
It is important for the users that they can execute their tasks with the new system. Therefore custom processes should be implemented in the new system as long as the process is used. This is a great opportunity to get rid of unused or rarely-used functionality.
The implementation of regression test and the documentation of functionality is another aspect of a successful project.
Integration of external systems
Existing integration with other systems might be the biggest issue. Be aware that new data and processes can affect salesforce and other integrated systems. Therefore the only chance to void issue is testing.
Besides from the technical perspective, salesforce is a socio-technical system and the user is one of the most important part of the system.
The merge of two orgs changes the way people are working. Therefore user-training and the distribution of information is important.