I had forgetten this thread-it was from awhile ago. I think @fia had the right idea in her decsription of the small conflict I observed being a sign of more long term annoyance for this particular pair. The INTJ was quite blunt, very 8-ish, while the INFJ was quite soft and sweet for an INFJ in the hard sciences. They had a history of constant misunderstanding between them, that led to intense dislike from the INFJ and disregard and arrogance form the INTJ.
I have to say that outside of this pair, most INFJ-INTJ interactions I have observed, go really well. I know quite a few INTJ-INFJ couples who are very happy together! It could be an enneatype diff that has a big impact.