Sequential vs Simultaneous Authentication Factors

I have noticed that most implementations of MFA involve a process where first the user enters username and password, and then the page changes/reloads so that those fields disappear, and then the user is presented with a new prompt for a TOTP token or sms code.

I have also noticed a trend recently where authentication will be a three screen process. In the first screen the user is prompted for username, then on the next screen they are prompted for …

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