I'd like to point out that because J/P has a different definition in socionics, dual partners, in MBTI terms will have ALL 4 letters reversed.
(Arguably, of course) There are several debates of what MBTI type corresponds to which socionics types. In socionics, J/P determines if a person's dominant(ego) function is a judging or perceiving function. This stands in contrast to the MBTI J/P in which it corresponds to which function receives the extraverted attitude. In the instance of extroverted types both systems will have the same function order because the dominant function is extroverted. In introverts, the auxiliary function is determined by the J/P preference in MBTI. In socionics, however, the J/P preference determines if the dominant function is a judging or perceiving function, not the auxiliary. As a result of all this, J/P is traditionally "flipped" in introverts between the two systems because of function order. (IE: INTP (MBTI) and INTj (socionics) have the same functions)
In Socionics, all letters except for J/P are changed for duals.
Therefore, MBTI "duals" would require ALL letters to change. (As one half of the dual would be introverted, thus requiring ONE of the J/P preferences to be changed)