I havent done the tut mate but I would have thought Jay copied the mesh in 'instance' so the 2 halves dont need to be joined? I could be wrong.(could some one clarify this?)

Or if they are meant to be joined and Jay has reflection turned on (in the move tool options)....you need to 'Polgygons>Mesh>Combine' first up...then you need to highlight the vertices where the join is...and then go 'Polygons>EditMesh>Merge and click on the little option box next to merge...set the value to 0.01 and hit apply...this will set this value from now on when you merge and also merge the verts....if they dont merge then increase this value a bit...selecting apply will set this value as an automatic distance.

cheers bullet