Final answer:
A critical consideration for ensuring successful communication between two microservices is establishing a well-defined API.
Step-by-step explanation:
A critical consideration for ensuring successful communication between two microservices is establishing a well-defined API. An API, or Application Programming Interface, defines a set of rules and protocols that allow different software systems to communicate with each other. By establishing a well-defined API, the microservices can communicate effectively by understanding the format and structure of the messages exchanged between them.