The Seelie Fey template can be applied to any fey creature of pure fey ancestry... Which means that, ironically, Unseelie fey are one of the few LA 0 ways to be a seelie fey. As seelie fey has no LA entry and advances "as the base creature", it has the same LA as the base creature (+0), as per the Monster Manual template rules. As such, unless I'm missing something, Unseelie Seelie Fey can be added to almost any creature at no LA, for scaling DR and SLAs, aura of courage, healing touch, turn undead, bonuses to lots of charisma based skills, and potentially a fly speed. So, is there anything in RAW that prevents a character with one Seelie parent and one Unseelie parent from taking both templates?
I'm thinking of using this for the jack-of-all trades Ranger/Bard/Sublime Chord/Folhucan Lyrist build over in the http://https://forums.giantitp.com/showthread.php?615106-PrCs-that-advance-Mystic-Ranger-spellcasting"]Mystic Ranger PrCs thread.