Organizing Surfaces in a List by Z Z value

Not understanding what I'm doing wrong. I've got surfaces sorted into branches by their face (North, East, South, West)

Within each face branch I would like to group the surfaces by level which I've tried to do by creating a unique set of z values of the centroids, and then using that member index map, pick them into sub branches. 

The result is breaking them into sub branches within the faces branch but I'm losing the organization of the faces. For instance, {0;0} is West Face level 1 and {0;1} is South Face Level 2 ect. I'm looking to make {0;0} West Face Level 1 and {0;1} West Face Level 2. {1;0} South Face Level 1 {1;1} South Face Level 2.

Thanks!