currentTab workaround

app.js

var tabGroup = Ti.UI.createTabGroup();

var win1 = Ti.UI.createWindow({
	title: 'Win 1',
	url: 'win1.js'
});

var tab1 = Ti.UI.createTab({
	title: 'Tab 1',
	window: win1
});
// pass a reference of the current tab to the window
win1.tab = tab1;

tabGroup.addTab(tab1);
tabGroup.open();

win1.js

var win = Ti.UI.currentWindow;
// get the reference to the tab here
var tab = win.tab;
win.backgroundColor = 'red';

var button1 = Ti.UI.createButton({
	title: "open window",
	height: 40,
	width: 200,
	top: 100
});
button1.addEventListener('click', function(e){
	var win2 = Ti.UI.createWindow({
		backgroundColor: 'blue'
	});
	tab.open(win2);
});
win.add(button1);