How to sync notification and ringtone volumes when pressing up/down buttons on Galaxy S6?
I don't think there is anything built in to the Android O/S to link/sync ringtone and notification volumes (for the Galaxy S6 anyways).

The best I've found (and currently using) is Volume Sync Pro by Plextab Software.
It does exactly what you want with the up/down volume rocker button. It's a paid app though.

I've tried these free apps and they did not work for me.


volume Control + by Coffee Break Apps
Volume Sync by Jesta's Funhouse