Due to on screen and novel evidence, Dual Lightning is exclusive to Ozai only.
Creating Lightning mid air while adjusting a fire jet to maintain the right altitude is unheard of.
Roku's identity as the Avatar was revealed on his 16th birthday. His Avatar training took 12 years.
This is him at 28 years old as a fully realized Avatar.
Fun Fact: Azula never faced Aang in a fair fight.
1. Fought an exhausted aang who was cutting metal with water bending.
2. Had more dai li agents for back up plus she was with zuko.
3. Fought Aang who went a day without sleep.