I'm not sure I want to allow it either, but I also think that, aside from the silliness of it, a Flaming Flaming Sword is fine. It's not essentially any different than having a Flaming Shocking Sword. On the other hand, a Wounding Wounding Longsword does strike me as really silly, but I could probably even justify that (I could imagine a +4 ability that did double of what wounding did, say, Eviscerating or something, and thats not any different than Wounding Wounding). Though there are some abilities I think obviously can't get added twice, like Ghost Touch or Brilliant Energy. I'm just not sure whether Spell Storing falls into the 'Why not?' or 'That makes no sense!' category.