Send logged in users to wp on a different sub domain

Once a user has logged into the site using the wp_login_form, is it possible to send them to the other sub domain keeping them logged in?

In order to do that either:
1. Your two WP installs share the same database then also have the same users table within the database. Then install a login redirect plugin http://wordpress.org/extend/plugins/search.php?q=login+redirect&sort=
or
2. From your WP in main site, create a network. Then create a subsite and import information from your second WP into that subsite. Then install a login redirect plugin http://wordpress.org/extend/plugins/search.php?q=login+redirect&sort=