You can use Authenticator Pro (android, opensesource) and Proton Pass, both let you copy the TOTP generation code to paste into another without problem. Both generate exact code
In fact that's how I am using them right now, with Authenticator Pro is my on-device, offline, encrypted backup offline backup TOTP for Pass.
I guess it is not as straight forward as export import as you hope, but it's not as bad as other options used to be.
Bitwarden has launched a new authenticator app ( bitwarden.com )
Bitwarden Authenticator is a standalone app that is available for everyone, even non-Bitwarden customers....