You'd have to burn the VIDEO_TS folder/files directly to the DVD+R DL disc, you wouldn't be able to make an image first as it would know the files fit on a single layer disc and just build a single layer image.
*ok, that isn't 100% true, you could adjust the single layer media size right down so it exceeds that and then builds an image for a DL disc. Just don't forget to revert the single layer media size when you're done.