How can one minikube cluster talk to another?

[Jules Lai]

I have two minikube clusters with different profiles; one for an ExpressJS server, another for a Rust server.
The ExpressJS calls the Rust server via gRPC.

I cannot figure out what end-point address to use for the Rust server.

When the ExpressJS is running locally outside the cluster as a normal app then localhost:40040 will work. But when running inside its minikube cluster then it errors, complaining that  localhost:40040 is missing http. So I use address http://localhost:40040 but then it won't connect "Error: 14 UNAVAILABLE: No connection established" I tried http:127.0.0.1:40040, same error.

So I am wondering how can one server inside a minikube cluster communicate to another server inside a different minikube?

Thanks any help,
Jules