While everyone might assume dual layer DVDs that hold 9.4GB of data are always going to be way better than 4.7GB DVD-R disks holding the same amount of data, this isn't necessarily true. A dual layer disk will hold two hours of video at the highest DVD data rate which is 9.6. To fit the same movie on one layer DVD-R, the data rate is cut in half. But it isn't like the quality is cut it half. In fact many store bought DVD movies are actually on single layer DVD disks, so if the movie is two hours, the data rate is half the max. Many others have a data rate somewhere between and the extra space is takes up by added DVD material. Just because the manufacturer of the DVD decided to use the maximum data rate with the movie, doesn't mean it is always going to look better. The encoding process plays a big part as does the quality of the original movie. In my opinion, some store bought DVDs with dramatically lower data rates than 9.6, sometimes way lower than 4.6, still look fantastic.
I've used things like DVD2one and DVD shrink with good results, even with movies three hours long. The better the original DVD looks, the better your results will be when you compress it to one DVD-R disk.