Hmmm I think scoring it might cause a few lumps and bumps under the new wallpaper.
One problem you might encounter is that the paste will soak through and lift the wall paper underneath but it should dry out again.
Best answer is do it properly and remove the old stuff - you know you want to ;O)
Best answer would be not to do it. As Quoi says it will probably cause some of it to lift and when you have 2 joins in same place it will just pull away. It will never look that good and you will problably end up doing it again causing a lot more work, mess and expense.
In the past, putting on two layers of wallpaper was quite deliberate. The first layer was of thin absolutely smooth paper which resembled brown wrapping paper. This was at that time considered the "Rolls Royce Treatment" in decorating.
We once moved into a house and found there were at least 5 layers of paper with paint over the layers on one room. It took 3 of us 4 days just to strip it off.
You can put paper on top of paper, but not on top of vinyl. ( no need to scrape) I have worked in houses where there were 22 layers of paper on the walls. Or possibly more - we lost count.