66+ OoW/DoD Spell Rune Quest questionFollow

Just wondering if the Shadowspine/muramite runes are interchangable? For instance I used Shadowspine runes for my first 3 level 66 spells. In order to get the fourth, do I have to use a Shadowspine rune, or can I just use one muramite rune? Or will Karsor just give me my first 66 spell again? just curious, thanks for the help

Biltene Flaymestryke
68 wiz

I used a combination, and whichever you use, you get the next spell in line. Yes, the turn in is to two different people.