Several times in your career you will face a situation wherein you will require to merge data of two salesforce org. Reasons could be any. However a clear plan on the course of action can save you from lots of future hassles and a successful implementation.
The first thing you can do is contact salesforce.com or your preferred consultant for a health check and analysis. You will also have to check to merge which org into which, for eg: which org has more user adoption, so all these analysis will help you in your strategy. Your company officials might push you for a speedy execution but you will have to explain them the pros and cons of an incorrect merger.
There are lots of things to consider Record types, Fields mapping, workflows, naming convention, duplicates etc.
Just being and Admin and having all the technical knowledge of Salesforce wouldn't suffice, you will have to talk to your employees, the actual salesforce users as their inputs will play a crucial role.
Consider some apps from Salesforce for data duplication prevention, an app may be to outline a flowchart or a project management app to give you clarity on the process. Plan this whole implementation in a project management software with clear milestones.
There can be other problems as handling CSV, Validation Rules, Salesforce system fields, which edition is in place? , API Limits etc
Mind It : Data Integrity is very very Important!!!!!!!!!!!!!!!!!!!!!
