The easiest way is to set the rule to execute on Insert, and then change the split-policy on the line to delete-insert-insert. This way when you do a split, what will really happen is 3 events
The original long line will be deleted. (if you have a delete AR it will trigger here)
the first segment will be inserted (to the left anchor) and attribute rule will be triggered
the second segment will be inserted (to the right of the split anchor) and attribute rules will be triggered on insert
all attributes are preserved from the original line (essentially duplicated)
you can handle the same thing with the existing split policy (update insert) its just little harder.
you can change the split policy from the properties of the class.