Expansion of FIDO standard and new updates for Microsoft passwordless solutions

"Multiple passwordless accounts in Microsoft Authenticator 
When we first introduced passwordless sign-in for Azure AD (work or school accounts), Microsoft Authenticator could only support one passwordless account at a time. Now that limitation has been removed and you can have as many as you want. iOS users will start to see this capability later this month and the feature will be available on Android afterwards."

👉 It works now (2022-08-01), I could not tell you since from which iOS Authenticator version, but, by accident I stumbled on it and did not get the "Your device is already registered" block, but there is now a list below 'Device registration' in the app. This allows you to register the device to multiple domains / tenants or for more than one account on the same tenant (though that remains one entry in the device registration list).

In other words. You can use the Microsoft Authenticator now with Passwordless for multiple accounts. (Experienced on iOS.)