app.js

var token = Titanium.App.Properties.getString('token');
if(token == null) {
  // show sign up screen
  var win = Titanium.UI.createWindow({
    url: 'main_windows/auth/choose.js',
    backgroundColor: '#fff'
  });
  
  win.open({
    delay: 1000,
    transition:Titanium.UI.iPhone.AnimationStyle.FLIP_FROM_LEFT
  });
} else {
  // show menu
}

choose.js

var win = Titanium.UI.currentWindow;

var buttonView = Titanium.UI.createView({
  backgroundColor: '#fff'
});

var signInButton = Titanium.UI.createButton({
  color: '#000',
  title: 'Sign in',
  color:'#000',
  top:110,
  width:301,
  height:57,
  font:{fontSize:20,fontWeight:'bold',fontFamily:'Helvetica Neue'}
});

var signUpButton = Titanium.UI.createButton({
  color: '#000',
  title: 'Sign up',
  top:210,
  width:301,
  height:57,
  font:{fontSize:20,fontWeight:'bold',fontFamily:'Helvetica Neue'}
});

buttonView.add(signInButton);
buttonView.add(signUpButton);
win.add(buttonView);