It doesn't make sense to try to "support multiple keys" for TOTP. You can copy-paste TOTP seeds if that's what you want and feel comfortable with, if the site tries to allow you to use any of N seeds they not only increase their system complexity they also reduce their security by a factor of N which makes no sense.
Edited to add: OK, Coinbase does now have U2F and they clearly state you can use "a maximum of 5 keys" which feels like that's enough.
https://support.kraken.com/hc/en-us/articles/360001363963-Yu...
It doesn't make sense to try to "support multiple keys" for TOTP. You can copy-paste TOTP seeds if that's what you want and feel comfortable with, if the site tries to allow you to use any of N seeds they not only increase their system complexity they also reduce their security by a factor of N which makes no sense.
Edited to add: OK, Coinbase does now have U2F and they clearly state you can use "a maximum of 5 keys" which feels like that's enough.