There's a protocol, FIDO Credential Exchange Protocol (CXP) which is currently at proposed standard status. It is supported by Apple and Google and some third party password managers (1Password, Bitwarden, and Dashlane). (1Password is kind of annoying though as its CXP export only supports exporting everything. There is no way as far as I can tell to export just a single item yet).
Once 1Password supports proper single export when I make a new passkey I'll store it there and later export it to Apple.
Meanwhile I simply make two passkeys. I've only run into I think two sites that supported passkeys but would not let me make two.
On most sites making a second passkey is as simply as going to your security settings, finding the passkey settings there, hitting the "add another passkey" link, and pointing your phone at the QR code it shows, and then on those phone choosing the password manager that you did not use for the first passkey.
