Of course it is impossible to have the same quality on A single layer disc as it is with a dual layer disc. But we are NOT talking about one single layer disc, we are talking about TWO. If you take a transfer and put it on to TWO single layer discs and then the same transfer and put it on one dual layer disc, the quality would be about the same between two single layer discs and one dual layer.

2 single layer = 4,7+4,7= 9,4GB
1 dual layer = 8,5 GB

So actually you will get 0,9 GB more on two single layer discs then on one dual layer.