I've tried this. When I did, Talkatone ceased functioning. When I contacted developer, he said that forwarding to your cell phone must be turned off in order for Talkatone to work. It may work just fine if I am using the GV app on the iPhone vs. Talkatone. I prefer the Talkatone interface and feature set, but if the GV app will allow simultaneous ringing, it may be the way to go.
Sorry, didn't understand what you were asking above.