If skirnir magus had a shield that was enhanced as a weapon and given the spell storing property, could he use his spellshield ability to store a second spell in the shield?
Spellshield:
Spellshield (Su)
At 7th level, as a standard action, a skirnir may store a magus spell in his shield by spending 1 point from his arcane pool per level of the spell. This functions as the spell storing weapon special ability, but activates only on a successful shield bash by the skirnir and is not limited to spells of 3rd level or less.
This ability replaces knowledge pool.
Spell storing weapon property:
A spell storing weapon allows a spellcaster to store a single targeted spell of up to 3rd level in the weapon. (The spell must have a casting time of 1 standard action.) Anytime the weapon strikes a creature and the creature takes damage from it, the weapon can immediately cast the spell on that creature as a free action if the wielder desires. (This special ability is an exception to the general rule that casting a spell from an item takes at least as long as casting that spell normally.) Once the spell has been cast from the weapon, a spellcaster can cast any other targeted spell of up to 3rd level into it. The weapon magically imparts to the wielder the name of the spell currently stored within it. A randomly rolled spell storing weapon has a 50% chance to have a spell stored in it already.
If skirnir magus had a shield that was enhanced as a weapon and given the spell storing property, could he use his spellshield ability to store a second spell in the shield?
I don't think so. Spellshield says as spell storing, but (exceptions). So, other than those exceptions (Which aren't really relevant) it should be like spell storing. And since weapons cannot possess the same special ability more than once, I think they don't stack.
I could see people running it the other way, though. Nasty, but doable.