Double layer disks are so expensive that it is hardly worth it to use them to back up movies.
I would suggest that you use the free DVD Shrink to shrink the movie to fit on one single layer DVD+R bitset to DVD-ROM. I doubt that you would be able to see any quality difference.
When the Japanese single layer FujiFilm 8X DVD+Rs go on sale at Best Buy you can get them for about 40 cents a piece. Today they generally sell for 80 cents a piece when not on sale.
The price is right to use DVD Shrink and the Japanese single layer Fujis.