Select DVD-5 mode in CloneDVD. Select the movie title and sound track that you want to keep (usually main movie and AC 3/6 audio). Now check the quality bar the bottom. If you see 100%, then there is no compression and the ENTIRE movie will fit into one DVD-5 disc. If you see 80%, then CloneDVD must compress the original DVD by 20% to fit to one DVD-5 disc. I usually split the movie at 80% picture quality.
To split the original movie, use the back button to go to the main movie selection screen and click on the scissor incon. Go to the ends of the orange bar to select the chapters you want burn to the DVD. These chapters will appear in the upper right, next to the movie title.