Thanks for your reply.
When I said simulate, I meant generating requests, like the ones you mentioned in the composer tab.
But my issues are a bit complex than regular requests. I have a client, and 2 different servers communicating between them.
My client cannot capture the calls between the servers as it only communicate with 1 server and with limited method requests.
To make things simpler, the client communicates with server #1, and server #1 sends requests to server #2 (which he then responses to server #1). In that fashion, the client communicates only with server #1.
I'd like to initiate requests from my client to server #1 but to manipulate the request from server #1 to server #2, and manipulate responses from server #2 to server #1. I only know how to generate requests and responses that are independent, but I don't really know how to do it that in an end to end scenario like I just described.