Quoting thephilwells from https://github.com/w3c/webdriver/issues/39:

Using the Java API, one should be able to do this

List<String> handles = driver.getWindowHandles();
driver.switchTo().window(handles[1]);

..and able to reliably expect that they are viewing the second-oldest open window. This matters most when more than two windows are open, as when one needs to open more than one new window from the original window.