I play a wizard 4/fighter 3/spellsword 1 (Complete Warrior 79-80). With a few more spellsword levels I'll gain the special ability channel spells. I already have a greatsword with the magic weapon special ability spell storing. Will the magic weapon special ability spell storing and the spellsword's special ability channel spell combine so that two spells can be used simultaneously on a single hit?
It seems you can release both spells in a single hit, the one that was stored in the weapon by a spellcaster and the one you channeled into the weapon with your spellsword class feature.
The two effects stack in so far as the weapon is able to hold both spells. There is no rule that precludes that.
Releasing the spell stored in the weapon is a free action and the description of free action says:
"You may also perform one or more free actions along with any other action, as your DM allows."
That means free actions are at your DM's discretion. He could rule out taking free actions along with the hit releasing the channeled spell. But you could take the swift action you have every round, that is guaranteed.
Releasing the spell channeled into the weapon with the spellsword class feature is part of the action of hitting, so it is not an action in itself:
"effects the next target that the spellsword successfully attacks with his weapon" Complete Warrior p. 80
Thus, it does not collide with releasing the stored spell. This reasoning does also apply to the Multichanneling class feature. In this manner a 10th level spellsword with a spell storing weapon can release three spells with a single hit.
The only limits: You cannot chose NOT to release the channeled spell, once you have channeled it into the weapon. If the effect of the stored spell is not applicable on the target of your hit, you have to refrain from using both spells. And this hit has to occur within 8 hours of channeling the spell into the weapon.