When two companies merge, the provisions of the Companies Act 2013 would apply. Besides that, in case you are registered as a factory as per the Factories Act 1948, the provisions on change of management would apply, and you need to nominate the Factory Occupier and Factory Manager as well as get the factory license amended.
In case you are a shop/establishment as per the Shop & Establishment Act, you have to inform the authorities concerned of the changes in composition. Additionally, an immediate task is to identify the positions that will become redundant; the plan to utilize them or to jettison them has to be prepared.
Most importantly, the task of HR is to prepare both the organization and the employees for the merger. Employees will have fears, apprehensions, and, more importantly, misgivings. It is necessary to communicate clearly and forthrightly the process of change and reassure the talents of continued care and support.