We do not support two factor with the native authentication. We recommend setting up SSO on NWC with your own federated authentication provider. Help guide on how to setup available here: https://help.nintex.com/en-US/nwc/Content/Settings/ConfigureSAML.htm
At present in Nintex Workflow Cloud, this is not possible.
Multi-factor authentication along with the ability to set the password strength for your organization is however on the identity management roadmap for next year.
I hope this helps.
Regards,
David