In a multi model situation, shouldn't LLM A talk to LLM B as a tool call via MCP? or would it talk to LLM B directly?

How can an LLM “talk” to another LLM, except by emitting tokens in its output stream?

You can name the mechanism whatever you want, but the models don’t have hands. Tool calling conventions (as a concept, or as a spec) is what gives the model hands!