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.
TOTP has always been a second password (heck, it's in the name). If you know the secret and the algorithm you can do the maths yourself in theory without needing any hardware, so in theory it can always be considered "something you know", even without all the syncing stuffs from Authy.
In any case I don't see how the Authy password can weaken TOTP. It's not like there's a webpage out there where you can enter the Authy password and it will give you back the TOTP secret for a specific user. It's only used to decrypt the TOTP secret if you choose to sync that secret to another new device, which again requires SMS verification, PLUS confirmation from an existing device, PLUS you need to have the sync capability setting enabled (so you can always sync the TOTP to your backup device first then disable the sync setting to prevent additional devices being synced).
It does seem like this is somewhat more secure, in some sense, but it weakens the security that TOTP is intended to provide.