Append mesh and fuse faces
 

Hello
I tried to follow this for my own practice: https://youtu.be/E7wn2TbZZ-c
I have attached pics of my sample case. I have an inter control volume with my sphere that I want to analyse. The outer control volume has the volume of the inner control volume removed.
I can merge the two fluid zones and get one cell zone. When I try to fuse adjacent faces of the two control volumes I get an error that there is a difference in the number of nodes. But, all involved faces have the same number of elements which has been enforced by edge sizing and face meshing.

Instead, I had to define an interface and work with that. Could someone tell me why I am having this simple issue?

Thanks