“if i just do a copy disc in nero” - as I said, Nero cant do that so the rest of your question is irrelevant.
If you copy the DVD to your HD with Decrypter, the IFO files will still be pointing to the layer break that was on the original DVD. When you burn this back with Nero, then Nero is able to sort that out and point to the new layer break point, as it will not be in the same possition as the original. Original DVD can have varying amounts of data on each layer (such as 4.3gb on layer 1 and 1.7gb on the 2nd) where as for burning to DVDR DL media, it has to be burnt equalually to both layers. In the above example it would be 3gb per layer. Nero seams to be able to sort all that out automatically.
Friends have tried DL media burning with Nero and there is only the one layer break seen.