It needs its own UV space within the 0-1 area-- not stacked. So some shifting around of other UV elements/islands would have to be done to fit it in there. Even if the UVs are identical, if the texture needs to be unique it can't be stacked.
Got it, thanks. Element selection is bothering me now =) Your hint about it being slower than calculation was right and sent me on a journey of poking around with element detection and reorganizing the whole thing. Expanding to element is slow, but does wonders with low amount of shells, say we have million polys unwrapped…
No, that's not it. It's being able to select an element in the UV window and have the corresponding element highlighted on the 3D object. If I click a vertex on my UV, it should highlight the same vertex in 3D view.
if it only tiles in one direction, you could use a non-square texture, like a 256x1024. Then you unwrap the unique elements inside of the uv borders and have the tiling elements cross it.