What version of Encore?
In CS3, you can give each chapter an end action (for individual play) and add all chapters to a single chapter playlist for the Play All function. In CS3, there are no limitations or restrictions on the use of chapter playlists in this way.
In 2.0 you can split your main timeline (using your NLE) into 2 or more less-than-18-chapter timelines and use chapter playlists again as described above. Or, you can stay in Encore and give each chapter its own playlist (for individual playback) and use the main timeline (with no chapter end actions) for the Play All function.
In 1.x, you will need separate timelines for each chapter regardless.
-Jeff
The Focal Easy Guide to Adobe Encore DVD 2.0