I was just reading something today that said that said that ENFJs and ENTJs are like mixing oil and water, not that are opposites, but that the ENxJ part of both of them makes them the dominant leader, which doesn't go well, because in their eyes, only one person is the leader..it's sort of like when two dogs meet, and they try to find the dominant male or female..well, maybe the same concept here, towards who the dominant leader is.
Also, the T and the F don't mix, necessarily, because their leading values are set of different levels.