They do stack, but (as far as I know) the bonus you get is only based on your base stats. For example, if you have a base 50 will (which includes your gear) and you cast calm the troops you will have 75 will. Then if you cast big juju (that's the 50% one right?) you will end up with 100 will, not 112.
I believe they are stackable, The way to check is to cast both of the powers and then hover your cursor over your character. You should see two different Increase Will IF you have a weapon with a Bonus from Will as the weapon you are using. Big Juju and Juju use the weapon you are holding to determine which attribute is increased.