Ok, seems I almost got it, but now the problem is, after i setup the material for that object, and these are the 2 result:
1)If I join them, both object takes the same material, so or they become both invisible, or they become both visible
2)If I don't join them, they act like something weird.
Maybe I'm too newbie in blender
Edit: I tried the second method you said, and seems like it keeps the height like it should be, I will do further tests.