You really, really need to have two different physical security keys. If you only rely on a smartphone as your key, and that phone is lost, stolen or dead, you're screwed. It can take several business days to get Google to manually establish your identity and reset your account.
So, I suggest that you go ahead and buy the Titan key bundle ($50), or two keys in your choice of USB interfaces (A or C), either from Google or from Yubico.