Dueling involves 2 people ... usually ... let's say at least 2 people.
The reason this is important is that what you do has to be relative to the other person in the equation - their personality, fighting style, physical size etc. You do what you do because it is that particular person facing you. If it was another person in front of you, you change your game to fight them.
An easy example would be noting the difference in tactics between fighting someone 6'5" (One of Sonny's students was this tall) vs someone left handed vs a Serrada player vs a high skilled VCKE player.
You are also part of the equation of course, with your own strengths and weaknesses, so again, relative to your opponent, you make choices regarding the best strategy to use (obviously the more options the better).
It's the same as dancing (or conversing*) with someone, the person standing in front of you is why you dance like you do - there are 2 people involved in the interaction, at least there should be (it's called playing with yourself otherwise), and dancing to your own beat with no regard for the other will get you 'killed' dueling if your opponent can read your game.
It's always a good idea to know who you are fighting, and in flow training, a way to find out who they are, to understand their rhythm, their personality, is to get out of your own head and body and into theirs.
What is their natural rhythm, cadence, tempo? Is there only one? More?
Can you find them?
What does this tell you about who they are, how they fight?
Can you put their rhythm into your repertoire to fight others?
Sonny called this 'Mirroring' (FYI: there are many different types of Mirroring, not just regarding rhythm) and it is a very important skill in our system.
* - (For those that are interested, the article I wrote a while back (jeez it can't be 7 years ago already ...) about flow: The Art of Conversation is here: http://www.visayaneskrima.org/articles.html )