Board Thread:General Discussion/@comment-4957813-20131130185522/@comment-5553481-20140717162507

The reason Zangetsu and Old Man were able to merge is because Ichigo is human as well.

Vizored were able to survive because they were placed in a HUMAN gigai, and were given a "medication" created from a QUINCY arrow. That's how Urahara 'balanced out' their souls.

Now, Quincy do not force a merger with Shinigami, they take their bankai and keep it WITHIN the medallion, it's not merging at all.