This scenario is not supported by the RadWindow control, because the modal and the maximized RadWindow is expected to be on the top of all the elements on the page. However, it can be achieved using the RadWindow's client-side API. For your convenience, I prepared a sample project that demonstrates how to implement the desired functionality. The idea is, when the second window is shown and the first one is maximized - you simply restore the first one; then in the OnClientClose event handler of the second window you maximize the first window.
I also included detailed comments in the source code that I believe will be of much help to you.
Please do not hesitate to contact us if you have any questions.