To use two-factor authentication you can use your Gmail, LinkedIn, Facebook or Salesforce account as a log in.
In the account of your choosing you can set up the two-factor authentication. It's important though that the email address of your SignRequest account is the same as the account with which you choose to log in. Otherwise you will create a new SignRequest account with a different email address.
We're currently working on adding two-factor authentication to SignRequest as well.