Merging IT Infrastructure and ISO 27001

To ensure your organization keeps compliant with ISO 27001 in this merging you should treat this merge as an implementation project with some adjustments:
1) reviewing ISMS basic framework (e.g., scope, objectives, organizational structure), considering the merged organizational context and requirements of interested parties;
2) review of risk assessment and treatment methodologies, to see which elements can be merged and which ones need to be kept separate;
3) review the risk assessment and define the updated risk treatment plan;
4) adjustment of implemented controls when necessary (e.g., policies and procedures documentation, acquisitions, etc.), as well as the implementation of new controls required due to the new merged context;
5) people training and awareness;
6) controls operation;
7) performance monitoring and measurement;
8) perform internal audit;
9) perform management critical review; and
10) address nonconformities, corrective actions, and opportunities for improvement.  

These articles will provide you with additional information:

• Three strategies for ISO 27001 implementation https://advisera.com/27001academy/knowledgebase/iso-27001-implementation-checklist/#options
• ISO 27001 implementation steps https://advisera.com/27001academy/knowledgebase/iso-27001-implementation-checklist/