Can you please tell me how to split a line or spline in order to cut it in different ways?
Thank you in advance
I think you want the edit nodes tool. See below.
Insert node (I) where you want to “cut”, then Break (B).
The Offset tool will split a line / curve into two parallel vectors / curves along its length:
If you want an identical copy of the entire shape to put on a different layer:
If neither of those work, we’ll need an example of what you want.