The PREACHER is a character in OMORI. He is the priest who takes care of the CHURCH in FARAWAY TOWN. The PREACHER is a kind-hearted and religious man. He is open-minded towards others and never judges someone based on simply appearances or rumors.  He is also very patient. He can be very perceptive, being capable of seeing when someone is troubled. 

WISE ROCK is a character and NPC in OMORI. It is an sentient rock that can give OMORI and friends any type of advice whenever they need it within a grotto in the VAST FOREST. WISE ROCK is a small gray rock covered in a few chunks of grass. It possesses two small eyes and a mouth. As shown through its wisdom and knowledge, the WISE ROCK is a humble individual when it comes to giving advice to those who need it. 

[descriptions pulled from the OMORI Wiki]

There is no deeper, super meaningful reason as to why whoever voices the PREACHER will also be voicing the WISE ROCK. We just thought it'd be funny. Give us your best interpretation of both!