There's no setting or simple tweak that I know of to allow you to do this though it would be a nice option to display multiple banners at once.  However, you could achieve your goal by ditching the banner skinobject and replacing it with your own custom display while still using the provided DNN banner configuration interface to manage your banners.

Check this helpful article by Patrick Santry (former DNN core team member) on creating your own custom display of DNN banners (it still uses the DNN banner system for banner management but it does NOT require you to modify any core DNN code):

   http://www.wwwcoder.com/Directory/tabid/68/type/art/site/2073/parentid/224/Default.aspx

You should be able to make minor modifications to the example code from the article to display more than one banner at a time.

Cheers-
-mamlin