Blade Storm (11th level): As long as you are armed with a melee weapon and are capable of making an opportunity attack, one adjacent enemy (your choice) takes damage equal to your Dexterity modifier at the end of your turn.
Twin-Blade Storm (16th level): As long as you are armed with a melee weapon and are capable of making an opportunity attack, two adjacent enemies (your choice) take lightning damage equal to your Dexterity modifier at the end of your turn.
These are different features and do not consume opportunity attacks (and in fact, cannot, as they occur on the player's turn.)
Does this mean that one adjacent enemy takes 2*Dex damage every round?