Simple Single-Context app

app.js

// namespace for the windows
var W = {};
// namespace for custom UI
var UI = {};
// store the tabs in this array for later use
var tabs = [];
// include the necessary files to run the app
Ti.include('ui.js');
Ti.include('other.js');
Ti.include('window1.js');
Ti.include('window2.js');
Ti.include('tabgroup.js');

W.TabGroup().open();

other.js

W.OtherWin = function(){
	var win = UI.Win();
	
	var label = Ti.UI.createLabel({
		backgroundColor:'purple',
		color:'black',
		text:'this is another window',
		width:'auto',
		height:'auto'
	});
	
	win.add(label);

	return win;
}

tabgroup.js

W.TabGroup = function(){
	var tabGroup = Ti.UI.createTabGroup();
	tabs[0] = Ti.UI.createTab({
		window:W.Window1(),
		title:'green'
	});
	tabs[1] = Ti.UI.createTab({
		window:W.Window2(),
		title:'blue'
	});
	tabGroup.addTab(tabs[0]);
	tabGroup.addTab(tabs[1]);
	return tabGroup;
};

ui.js

UI.Win = function(a){
	a = a || {};
	var win = Ti.UI.createWindow(a);
	win.orientationModes = [1,2,3,4];	
	return win;
}

window1.js

W.Window1 = function(){
	var win = UI.Win({
		backgroundColor:'#999'
	});
	
	var button = Ti.UI.createButton({
		backgroundColor:'green',
		color:'black',
		title:'this is window 1',
		width:100,
		height:40
	});
	button.addEventListener('click', function(){
		tabs[0].open(W.OtherWin());
	});
	win.add(button);
	return win;
};

window2.js

W.Window2 = function(){
	var win = UI.Win();
	
	var label = Ti.UI.createLabel({
		backgroundColor:'blue',
		color:'black',
		text:'this is window 2',
		width:'auto',
		height:'auto'
	});
	
	win.add(label);
	
	return win;
}

Should there not be a

return win;

on Window1.js?

Yes, sorry. It's fixed now.