Brad Frost on Clever Login Forms
Don't be too clever with login forms. If you're going to look for a time to follow established patterns, it's probably here. @brad_frost gets a lot right in this piece. bradfrost.com
One thing here—splitting login to multiple pages is almost certainly being done for organizations with SSO/SAML on enabled. There are alternatives to it, but the app is waiting for an email to determine where to send the user.