Let me clear up a little bit more how the rule is supposed to work:
- It is a free action.
- It affects a single target (Target: one creature). That means you lower the hp of yourself, you gain the attack bonus against only a single target or you gain the damage bonus against only a single target.
- You can drop below zero hp by using this ability (not 100% sure about this).
Indeed the defender gets more due to having more healing surges than the striker but if that is an issue then it is still an issue imho even without my houserule...