Xnormal baking issue

Walkion

Ok, hi everyone, im coming here for help on last resort before going crazy on a normal baking process.
I'll be as clear as i can, I have a Low Triangulated mesh with a cage, both saved as fbx. I have a High poly mesh done on Zbrush and exported as Obj. Now when i input all the files in the correct slots in Xnormal, thats what im getting : http://s23.postimg.org/vsfa6escb/Normal_normals.png

 I was thinking of a cage issue so i checked everything ( unified normals, smoothing groups cleared, pivot set to origin, reset xfrom done ) . But either way i just keep getting the same result. Is there anything i'm doing wrong? I tried baking with quad meshes but its doing the same thing. When i do the baking on zbrush, i also have those weird rectangle shape that looks just like my low polygon topology .... .
I guess it must be a stupid thing but i really cant see what im doing wrong. I even tried with a higher subdivided low poly to see if the previous low poly was indeed to low to bake a correct normal, it improve things a bit but i still have those rectangle shape anyway, only smaller . Can someone help me on this one please ?
I have these two pictures showing the low and high in zbrush if it can help somehow : 

http://s15.postimg.org/u4gmyffh7/Zbrush_High.jpg
http://s15.postimg.org/cjnb0syl7/ZBrush_Low.jpg

Walkion

Oh one more thing, my uvs are sliced in two indentical shells and stacked on top of each others for Uv map optimisation, i do believe that could be a critical information

vertex_

You are going to get bad results from baking overlapping UVs, bake one side first and then duplicate. Otherwise I would say that you have not properly smoothed your low poly judging from the map output and your low poly model.

Add3r

Both points I think that Vertex_ pointed out, are spot on.  Try both, and get back to us Should resolve your main issues.  Also be sure to harden edges that are near 90 degrees or more, as well as the edges that are UV shell borders (where the edges are split), on your lowpoly before the bake.  Smooth all other edges. More info on proper lowpoly bake techniques are in the Polycount wiki.  

Walkion

Thanks a lot to both of you, i was indeed thinking the overlapping would be one of the problem. Concerning the soft/hard edges i think i just misunderstood the guideline i followed. It were specified to clear all smoothing on the mesh in order to make the baking work properly but what you are saying is completely logical so i'll do as you said. I try all this and i get back to you. Thanks a lot really !

Walkion

Ok you are the best, it seems the overlapping wasnt the guilty one here ( lucky i guess ). The smooth/hard edges in the other hand, once corrected, got rid of all the issues i had. That was indeed simple, i feel a bit ashamed ahah, but better to understand it at some point than to stay stupid ahah.
Thanks again guys, have a nice day !