An ISTJ/INFJ pair? That is a strange one.
I only know one ISTJ as a definite ISTJ, due to the fact that I typed him, and he strikes me as the type of fellow who is absolutely repulsed by anything NF. He and I are very compatible (company wise), of course, but an NF, not so much.
He is very sensible and fact oriented, collecting facts for the sake of knowing them, a trait which I find somewhat irritating, although I must admit that his ability to store, and even more importantly, to recall information is commendable and almost admirable. Emotionally speaking, not that we ever exchange anything emotionally, he is, like me, very detached, and does not enjoy the lofty emotional air of the typical NF, probably due to his inability to understand them.