Well, it can't.
Recently I've published an update of the Advanced Rest Client. New version has been upgraded to Chrome app (from so called legacy app) because old ones are deprecated in Chrome and the support for them has been removed in Chrome Web Store.
As an author there was very little I could do in this situation. I needed to upgrade the app so it still can be published Chrome Web Store. Many of you complains that the app no longer opens in Chrome tab but instead in new window. This is how Chrome apps behave and there's no going back. You can always discuss the issue with the Chromium team.
From my side I can only give you advice to open new instance of the app if you need more than one window:
Thanks for using the app and as always happy RESTing!