There actually is a potential risk of surfing with a browser with Tor and one without Tor at the same time, if you have a circuit to the a site via Tor and also connect to it without Tor, if your internet dies both connections will break at the same time and the site can correlate it. So in summary *Using a Tor browser and non Tor browser at the same time? Safe. *Connecting to the same server with each of the browsers simultaneously? Potentially not safe, if your connection to the internet dies