Implementing a Symfony2 single-sign-on
In order to get multidomain SSO you can implement a SAML solution. Take a look on simplesamlphp software and in this extension for symfony1 https://github.com/chtitux/sfSAMLPlugin
To resolve the OTP issue you can use for example LinOTP2 witch is compatible with simplesamlphp: https://github.com/lsexperts/simplesamlphp-linotp