when you have Crusader skill LVL 1 and a weapon with Holy 1, the extra damage won't stack. I expected 2+2 = 4 extra damage but I only deal 2 extra damage. Is that planned that way?
First, the extra damage from Holy and Crusader applies only to undead enemy types. Just double checking. The stun chance is against all enemy types.
Second, Holy and Crusader add damage in two different ways. Holy, and all basic attribute types (Holy, Orc Slayer, Beast Slayer etc) add basic non-elemental damage to your attack. Crusader adds spirit damage as an elemental add-on. So Crusader's damage appears on it's own line in the combat log and on screen. Holy's damage is blended in with your basic attack damage.
With Holy I and Crusader I, you should notice 2 extra damage in your basic attack line and 2 spirit damage. Double check if that's not happening.