Communicating with multiple Espruino devices

On the master, it's just:

USB.setConsole(true); // make sure console stays off Serial1
var line = "";
Serial1.on('data', function(d) {
  line += d;
  i= line.indexOf("\n");
  while (i>=0) {
    console.log(line.substr(0,i)); // do stuff with data
    line = line.substr(i+1);
    i = line.indexOf("\n");
  }
});

And on the slave, make sure echo(0) is set to avoid the console outputting a load of extra control characters, then just call console.log(..) to return the data you want.