ICE modeling - merge uv projections

[mindFreeArtist]

Hi all.
I started to learn ice modeling and I got stuck at the moment when it came to uv's.
Simple scene - one polycube.
I created two planar projections - XY and YZ.
These projections work well individually but how to merge them together.
I tried to understand how it works on a cylinder from the sample scene "Modeling_Simple_Copy_Merge" but it's very complicated for me to understand.
Maybe someone can explain on a simple example.
Thanks.