The Thinking and Feeling pair of functions is bound together in the same way the Sensing and Intuition pair is. The two functions in a pair always go up against each or fight for the domination if you like. If one of the functions in a pair appears to be Dominant function then the other function in the same pair is Inferior and is well suppressed by the Dominant function, and the domination of the Dominant function is obvious. However, if one of the functions in a pair is Auxiliary, then the other would be Tertiary, which represents the Hidden Agenda. The Auxiliary and Tertiary combination of functions provokes a war between these functions, because there is no clear dominator. On one hand the Auxiliary function is much stronger than Tertiary, however the Tertiary function is the Hidden Agenda and is extremely important. So what happens in reality is that a person find it difficult to separate these two functions in order to give them clear preference, the functions always appear to be mixed up together to a certain degree.
So, if a person's Auxiliary and Tertiary functions are Feeling and Thinking (or Thinking and Feeling), then such person find it hard to keep these two in peace and may find it difficult to decide whether they are F or T. The Dominant and Inferior in this case would be Sensing and Intuition (or Intuition and Sensing), and should be quite easy to separate. Because the Dominant function is Perceiving in this case, the person would be Socionics Perceiving type (XXXp). So if MBTI INTJ person is more uncertain about being T or F than S or N i.e. IN(T/F)J or INxJ, then he or she will correspond to Socionics INTp.
If a person's Auxiliary and Tertiary functions are Sensing and Intuition (or Intuition and Sensing), then such person may find it difficult to keep these two in friendship and may find it hard to decide whether they are S or N. The Dominant and Inferior in this case would be Feeling and Thinking (or Thinking and Feeling), and should be quite easy to divide. Because in this case the Dominant function is Judging, the person would be Socionics Judging type (XXXj). So if MBTI INTJ person is more unsure about being S or N than T or F i.e. I(S/N)TJ or IxTJ, then he or she will correspond to Socionics INTj.
So here it is all of the above simplified:
Unsure about being T or F -> you are Socionics XXXp
Unsure about being S or N -> you are Socionics XXXj