You are going to have to login to seconddomainexample.com separately so the user can get the view_unpublished permission in that Context. PHP Sessions cannot be shared across domains, so it is not possible to Preview unpublished documents on other domains without logging into those domains specifically. You could do this across sub-domains with the proper session_cookie_domain setting which prefixes the domain with a ., e.g. .domainexample.com — then you could preview www.domainexample.com, sub1.domainexample.com, etc. as they would all share the session cookie.
How would i do this proper session_cookie_domain setting?
can you give an example?