Geimund, and his brother Gukimir are, as with all prospective Knights of the Nine, essential until recruited. When you talk to them they will ask if they can join. Say yes, and when you leave conversation they will both walk into the Priory. If they get inside they don the Crusaders equipment and are no longer essential. So, when you exit conversation with one brother, talk to the other before he gets inside, and you will be able to tell him to follow you. He stays essential.
Then if you have a high enough conjuration and armourer skills, you can reverse pickpocket bound armour so he looks more substantial.