Getting back to the MBTI/Enneagram connection...
Type theory is about perception and judgment--how people take in information and make decisions.
The Enneagram is about our defense systems--the mechanisms we use to protect ourselves from the world. I'm an INFJ/E7--I use optimism as a defense system and it becomes dysfunctional when I lapse into "I don't have to do anything proactively in this situation because it will all work out." INFJ/E3s, a more "common" combination, fall into perfectionism as in "it will all work out if I just do it all correctly".
As I understand it, each of the 16 types can have any of the 9 Enneagram defense systems, making for quite the variety.
The best resource on this I know is Pat Wyman's "Three keys to Self-Understanding" published by CAPT. Center for Applications of Psychological Type: MBTI and Archetype Training, Books, Research, Support Materials for Myers-Briggs Professionals and Users - Capt.org Home Page. In addition,the Association for Psychological Type, Association for Psychological Type International will be offering an online class on type and Enneagram, facilitated by Pat, this September. She's used the combinations in counseling women for years, helping them recognize when they're using their detrimental defense systems so they can turn around and use their healthy type preferences.