I usually take 2FA to mean that you have to use two of (something you have, something you know, or something you are.) If the "2FA secret" (TOTP secret?) is stored on multiple devices it doesn't actually prove ownership of "something you have" it's effectively no different from a password stored within a password manager which is considered simply "something you know." So basically the TOTP secret is a second password with some obfuscation that protects the password. But software running on one of your devices could easily steal the secret.

It does seem like this is somewhat more secure, in some sense, but it weakens the security that TOTP is intended to provide.