The only way to resolve this at the moment is through a gimmicky solution: opening 2 instances of address book side by side.
Personally I have just restored SL and abandoned Lion as I could not stand the new address book being a heavy user of the three pane functionality. There is no way of restoring it in Lion. It is really puzzling to see how Apple managed to make the already worse application in Mac even more abysmal.
In address book I would be looking to resolve these issues - there is a substantial worsening of tools in this new version of the app:
1- Revert to original 3 pane view (was available in SL)
2-Allow creation of new contacts directly in a group (was available in SL)
3- Restore the counter present for every folder to give number of entries in a given group (now you have to scroll to the bottom of the group to see it) (was available in SL)
4- Restore the icon to distinguish entries between company/person (was available in SL)
5- Possibility of removing of alphabetical letter grouping of entries
6- Option to change leather skin (revert to SL clean look)
and desirable improvements:
7-Allow font selection to facilitate reading and page display and reduce waste of space (was not available in SL)
8-Option to display contacts in table view for quick bulk editing (was not available in SL)
9-Show which (smart) group each contact belongs to on the contact’s page (was not available in SL)
10-Facilitate the creation of a new contact from the same company without having to copy across most details (was not available in SL)