Are the only things different about the two are:
Infj's show their true feelings when they are alone when someone that they can trust, where as the Intj's will hide their feelings all the time.
And that Infj's gravitate toward english and arts (I have a strong Ti, so I think that's why I'm so good at math and science) while the Intj's gravitate toward science and technology? (This is why I thought I was an Intj when I first started, my T/F scale is near the middle)
These are the only things right now that I can pull from off the top of my head.
Would anyone else care to elaborate?