Staff of Defense class feature:
...once per encounter as an immediate interrupt, you gain a bonus to defense against one attack equal to your Constitution modifier. You can declare the bonus after the Dungeon Master has already told you the damage.
Shield Wizard Utility 2:
Immediate Interrupt Personal
Tigger: You are hit by an attack
Effect: You gain a +4 power bonues to AC and Reflex defense until the end of your next turn.
Could you activate these powers against the same triggering attack and thus stack them against the same attack?