I'm doing 10' spacing and know going in, it will be tough to keep airflow through the canopies. In the event you're planting even closer, I would recommend just approach grafting the two into a single tree, and having a multi-graft canopy. That should be pretty manageable versus 5' trunk to trunk. Even having a single tree in a space that tight will be a pruning effort, but for annona, it's worth trying.