The only way to do this would be to use the WordPress multisite.
Have your main site as www.yoursite.com
then the secondary site as subsite.yoursite.com or www.yoursite.com/subsite
Using WordPress multisite allows you to have the same users across both sites so they stay logged in when moving between the 2.
You can't combine both themes together into 'one site' per say. Not without a tonne of coding.
Wordpress Multisite with user management plugin is the way to go.
*source: from experience of running a classipress & vantage multisite.