martillo said:But you can freely choose to put the first referential to one or the other twin, and the results are opposite!
This implies a contradiction!
One of the twins must be affected by a force and be accelerated in order for the twins to be united again. During the acceleration, one of the twins is not in an inertial frame of reference, so you cannot freely choose between the frames.
No contradiction.
And if both of the twins experienced a equal acceleration then there would be no difference in their ages.
Do some research on the so-called twins paradox and better yet, do some research on special relativity and how it can be used.