Benefit: When you use your dragon breath power, you can mix any two of the damage types to which you have access. Your dragon breath deals both types of damage. This doesn't increase the amount of damage you deal, but a creature must be able to resist or be immune to both damage types to reduce or ignore the damage. If a creature is vulnerable to either damage type, it is considered to be vulnerable to damage from your dragon breath.
For instance, if you chose the fire damage type at character creation and chose lightning when you took the Adaptable Breath feat, your dragon breath deals fire damage and lightning damage. A creature immune to fire but vulnerable to lightning is considered to be vulnerable to your dragon breath.[Dr365:28]