Isn’t the value of two factor auth that it requires a physical device (your phone or computer) with the auth key to authenticate you? Then why don’t many two factor auth apps seem to support syncing? If it’s fine to do so, are there any open source cross platform apps that sync keys?
There is the completely open source https://standardnotes.com/ which would support that via their syncing and the authenticator note type.
Unfortunately it does not look like their free plan allows you to use that note type. So could also host it yourself though (and pay for the premium token their or hack it out - it’s foss). Have never done that myself though