Shouldn't be too hard to imagine what the effects would be. The MBTI refers to the cognitive processes, which are basically the mechanics of your mind. The tools your rely on and the way they are callobrated. On the other hand, the instinctual stackings of the Enneagram refer to your concerns, pre-occupations, motives, etc. So the two mix easily. Just imagine someone takling the approach typical of their MB in efforts to work-out what they are concerned with according to the stacking. No aspects of the two has any conflicting overlap, so I don't know how to described it further without just sounding really obvious and redundant.
In case you're wondering, I've looked into this (even with a thread I made), and I've found no meaningful correlation between MB type and instinctual stacking. But, since I pointed out that the subject of the two systems have virtually no overlap, it's not surpising that there isn't a correlation.