"One on one" usually implies an adversarial interaction.
For example, a "one on one basketball game" or a "one on one interview."
It means one person is trying to do something to the other, like...
There is only one one in one one, but there are two ones in eleven. And there are no "ones" in one one if you are counting the "s". so there is one, one in one :)
1 plus 1 = 2 because 1 thing and another thing is 2 thingsthink abstract; like (example) 1 sandpile + 1 sandpile = 1 sandpile. Or 1 glass of water plus one glass of water = 1 bigger glass of water.