My servers beg to differ.

If your servers are making HTTP requests, then they are clients in the context of those requests.

So send a request in the opposite direction.

Now both are servers, and they're communicating with servers. Server to server communication on http.

(What would "server/server" even mean if you're making that distinction? Two communicating parties that only receive data, never sending it?)

Something like database replication where they’re equal peers.

Or an IRC network (the server to server parts of it).