Use two Portal Themes assigned to one user

Hello experts!

My idea:

If user A logs on into SAP NW Portal (AJAX Framework), he sees a company theme. Now, if he clicks on a tab in the first navigation level, its for example a subsidiary company, the theme for the subsidiary company should appear.

If I could use a kind of variables at the desktoprules, this could work - but at the moment I have no ideas.

Are there any experiences for that issue?

best regards,

Martin