User blog comment:Water Dragon Slayer/Chapter 348 - The Demon's Back/@comment-33019320-20130823091518/@comment-7418318-20130823102032

With the way the two bond with their spirits not just by ownership but also by friendship I doubt they'd ever share their keys in anything short of a really bad emergency (like say.... and army Dragons coming through a portal to the past perhaps?).

But what does this matter for the question at hand? They can be in the same guild without sharing their keys just fine.