In the event of a fire at the main data center, the immediate action should be to review and engage the disaster recovery plan. This is to ensure the continuity of business operations. The CIO should coordinate with IT leaders from both companies to ensure a unified response.
Assessing the damage and planning for recovery are crucial, and leveraging the expertise from both companies can help streamline the process.

Explanation
A Business Continuity Plan (BCP) is a set of policies and procedures that outline how an organization should respond to and recover from disruptions [1]. It is designed to ensure that critical operations and services can be quickly restored and maintained, and should include steps to identify risks, develop plans to mitigate those risks, and detail the procedures to be followed in the event of a disruption. Resources:
CompTIA Advanced Security Practitioner (CASP+) Study Guide, Chapter 4: "Business Continuity Planning," Wiley,
2018. https://www.wiley.com/en-us/CompTIA+Advanced+Security+Practitioner+CASP%2B+Study+Guide%2C

Implementing a multicloud provider solution would be the best option to ensure resiliency and meet SLA requirements in the event of a CSP incident. A multicloud provider solution involves using multiple cloud service providers to host and manage different parts of an organization's infrastructure and applications. This approach allows the organization to spread its workloads across multiple providers, providing increased resilience and the ability to continue operations in the event that one provider experiences an incident. In addition, using multiple providers can also help to reduce the risk of vendor lock-in, allowing the organization to more easily switch providers if needed.