Why doesn't this hit the callback for a2?

foo.js

var express    = require('express')
var a1 = express();
var a2 = express();

a1.listen(3000, function(err){
  if(err){ console.error('a1 ERROR!!'); }
  else{ console.log('Server a1 started on port 3000'); }
});


a2.listen(3000, function(err){
  //expect to log 'a2 ERROR!!'
  if(err){ console.error('a2 ERROR!!'); }
  else{ console.log('Server a2 started on port 3000'); }
});

output.txt

Server a1 started on port 3000
events.js:141
      throw er; // Unhandled 'error' event
      ^

Error: listen EADDRINUSE :::3000
    at Object.exports._errnoException (util.js:860:11)
    at exports._exceptionWithHostPort (util.js:883:20)
    at Server._listen2 (net.js:1234:14)
    at listen (net.js:1270:10)
    at Server.listen (net.js:1366:5)
    at EventEmitter.listen (/Users/sequoia/projects/code-along/my-express-app/node_modules/express/lib/application.js:617:24)
    at Object.<anonymous> (/Users/sequoia/projects/code-along/my-express-app/foo.js:11:4)
    at Module._compile (module.js:425:26)
    at Object.Module._extensions..js (module.js:432:10)
    at Module.load (module.js:356:32)