SmarterMail does already have this functionality, though it's not laid out as you request here. You can have multiple contact lists in your account and select which to view by using the Contact selector at the top of the navigation pane. (The fact that you can't view your GAL and another contact list together is not actually a bug in the software. Due to the way the Global Address List is generated, it cannot be overlaid with the default Contact list or shared contacts. However, overlay should work as expected in other sections that offer similar functionality, like your calendar, tasks, notes, etc.)
Aside from looking similar to the Email section, can I ask you why you prefer the folder pane to the current contact selector? I can see how the separate pane would allow you to drag and drop contacts among your lists; however, I can't think of other reasons this type of layout would be more beneficial. (I was actually surprised to find there is not a Select All checkbox on the contacts pane, though this is something that WILL be included in the next version.) Personally though, I think I would feel the folder pane would take up unnecessary space for something that can be handled by a dropdown menu like it's currently employed.
As it is, the Email section is actually the odd one out in terms of layout. The other collaboration sections that allow you to have multiple lists or shared resources -- Calendar, Contacts, Tasks, Notes -- all work similarly, where the lists you view are chosen using the selector above the nav pane.