Multiple conditions in Go IF statement

The idea is to do something only on Linux or MacOS. In C I'd check for OS type roughly in the following way:

#if defined(__linux) || defined(__APPLE__)
    printf("OK\n");
#else
    printf("NOT OK\n");
#endif

But in Go I have to do the same thing using AND operator instead of OR:

if runtime.GOOS == "linux" && runtime.GOOS == "darwin" {
    /* do something*/
} else {
    panic(fmt.Sprintf("Not yet implemented for %s", runtime.GOOS))
}

Quick documentation search did not give me any clear explanation on why if statement in Go behaves this way.

Can anyone please explain what am I missing?

Thank you.