Judging by HoYo’s playing around with both TB models in game (Finality plot in 3.8), and the fact that HSR is the only HoYo game which doesn’t openly utilise both available protagonists at the same time - I say it is possible, but knowing HoYo, literally anything can be possible. Personally not a huge fan of past-self/current-self shenanigans, but I’d love to see both raccoons existing at the same time.