Since you already have a polyline, you can perform one-sided buffering if you have an Advanced license.
If you have it, you simply need to specify the side of the polyline to buffer and the end type required.
Buffer—Help | Documentation
It would make it simpler if you dissolved/unioned the entire polyline to be one feature so you only have to deal with 2 ends.
Barring the required license, you are stuck with a standard buffer and splitting it with an extended version of your ridge line as suggested