How to un-link notification volume and phone ringer volume?

I love how I can set different tones for different notifications - I have a separate tone for phone ringer, text messages, work email, and Yahoo! mail so I can tell without looking what just came into the phone. BUT - there does not seem to be a way to set the volume level of the notifications (texts/emails) separate from the volume level of the phone ring tone. For instance - I can set the ring tone high, then I can go in and set the notification volume low. But if I go back and check the ring tone volume I see it's now set low - where I had the notification volume. Changing one level seems to automatically change the other level, and I can't seem to figure out how to un-link them. Does anyone know if this can be done? I would LOVE to have the ring tone level high, but the notification level low...