It's technologically safe, ie TorBrowser isn't going to leak info to a clearnet browser on its own. However, the potential for human error increases. If you're accessing Google over Tor and type a search in but accidentally type it into the clearnet browser, you have leaked info. It could potentially deanonymize you. As always, your behavior is half of your protection, and there's nothing TorBrowser can do for you if you behave in unsafe ways.