Yes, you can keep them on the same device if you choose to.

Or not. You decide how much effort you want and where you want to place the convenience vs security slider.

Yes, if you keep both factors not only on the same device but in the same password manager, then both factors essentially combine into nothing but a longer password.

I did say from the very first, that the seeds are nothing other than another password.

Except there is still at least one difference which I will say for at least the 3rd time... the totp secret is not transmitted over the wire when it is used, the password is. That is actually a significant improvement all by itself even if you do everything else the easy less secure way.

And you do not have to store the seeds the convenient less secure way. You can have them in a different password app with a different master password on the same device, or on seperate devices, or in seperate physical forms. You can store them any way you want, securely, or less securely.

The point is that even while opting to do things all the very secure way, you are still not locked out of anything when a single special device breaks, because you are not limited to only keeping a single copy of the seeds or the emergency passwords in a single place like on a single device or a single piece of paper.

You are free to address any "but what about" questions you decide you care about in any way you feel like.

The only way you were ever screwed is by the fact that the first time you set up 2fa for any site, most sites don't explain the actual mechanics but just walk you through a sequence of actions to perform without telling you what they actually did, and so at the end of following those directions you ARE left with the seeds only stored in a single place. And in the particular case of Google Authenticator, stored in a more or less inaccessible place in some android sqlite file you can't even manually get to without rooting your phone probably. And were never even told about the seed value at all. You were given those emergency passwords instead.

That does leave you with a single precious drvice that must not break or be lost. But the problem is only a combination of those bad directions given by websites, and the limitations of one particular totp app when that app didn't happen to display or export or cloud-backup the seeds until recently.

Even now Googles answer is a crap answer, because Google can see the codes unencrypted on their server, and Google can kill your entire gooogle account at sny time and you lose everything, email, drive , everything, instantly, no human to argue with. That is why I said even today I still would not use Google Authenticator for totp.

Except even in that one worst case, you still had the emergency passwords, which you were always free to keep in whatever way works for you. There is no single thing you must or must not do, there is only what kinds of problems are the worst problems for you.

Example: if what you are most concerned about is someone else getting ahold of a copy of those emergency passwords, then you want to have very few copies of them and they should be off-line and inconvenient to access. IE a printed hard copy in a safe deposit box in switzerland.

If what you are most concerned about is accidentally destroying your life savings by losing the password and the investment site has no further way to let you prove your ownership, then keep 10 copies in 10 different physical forms and places so that no matter what happens, you will always be able to access at least one of them. One on goggle drive, one on someone else's google drive in case yours is killed, one on onedrive, one on paper at home, one on paper in your wallet, one on your previous phonr that you don't use but still works, etc etc.

You pick whichever is your biggest priority, and address that need however you want, from pure convenience to pure security and all possible points in between. The convenient way has security downsides. The secure way has convenience downsides. But you are not forced to live with the downsides of either the convenient way or the secure way.