I think that actually its kind of a strange trick in nature that there's such a close alignment among the functions of our dual and conflictor. Granted this is debatable but to explain and speaking from a personal vantage point for example:
In the case of ISTJ vs INTP their conscious thinking functions oppose one another as well as the placement. Thinking in a dry theoretical textbook sense being the ISTPs primary mode of consciousness whereas for the ISTJ it is sencondary to sensing falling into a catagory I've seen described at the level where we take in information.
Their intuition also opposes in orientation (ne vs ni) and location...ni being the tertiary or third function in the ISTP that ive seen described as the level at which we make decisions.
The ISTJ, however is using ne in the fourth function or the or how we arrange our lives. It makes sense if you think of an inspector type in police work or i could see it going with accounting matching up with its duals system level...and it pairs well with the ti at the fourth level of the ENFJ representing structure and the crux of their "conscientious" orientation to life.
Anyway ... So yes they are very different, but I think people fall into a trap with their conflictors sometimes, because they can't distinguish between the two and its a temptation to hook up with your conflictor and to try to change them. It doesn't have to be romantic either it can be familial or professional or philosophical. What we can gather from socionics is that this endeavor is fruitless and we can avoid to much dependancy on our conflictors so as not to set ourselves up for failure on an emotional mental professional or private manner.
I think, however that since both are sensors (in this case) and both are introverts that it can be hard to tell. Today I was thinking of the difference between the two in those I've known in reality or on the computer...And I think both INTPs and INTJs can have a stern stare to them. I think also that both contain expiditor tendancies, but that in an ISTP its obviously more pronounced, but that there are elements of the ESTP running through the the kind of Model B section of the ISTJ. I look at type, now, as a primary set of four functions that are utilized differently, and a secondary less conscious set of four functions that we utiltize more privately and uncsonsciously which are the mirror of our conscious awareness.
So you could say that both an ISTJ and an ISTP are playful due to the artisan qualties, but that ISTPs are outwardly playful whereas ISTJs are privately playful. ISTPs are probably also more privately (or unconsciously) logistical and literally more supervisory (the model b ESTJ mirror part of their identity). If we can accept the idea that everyone is essentially their primary identity *and* the mirror type of that type.
Anyway I hope to speculate further and hope the topic is beneficial to someone out there...