Is it possible to have, for exemple, 1 iron halo and one storm shield, or 1 storm shield and the normal shield... do they stack (you have to roll the 2) o only the best of 2?

From page 166 of the Deathwatch Core Rulebook:

"A character may only benefit from one field at a time, regardless of how many different fields he may have equipped."

The only exception to this that I'm aware of is the "Force Field" built-into Terminator Armour, and I'm not even sure that qualifies as an exception. As a GM, I'd say the Terminator Armour still applies since it isn't technically a force field, but rather representative of attacks bouncing off the armour's hardest surfaces. Of course, that's just my call, others may have different views.

Ok thank you. I was making some test marines for my new game and i ran into this: what if they wanted to become impossible to kill with 3 force fields...

Now i know its not possible

Termie armor is not a force field per se. You can have a force field and termie armor. Tabletop does it all the time and in the fluff...ie storm shields and termie armor, and the termie chappy with rosarius.

The Crux Terminatus on every Terminator’s shoulder plate serves as a ward...and...provides the same chance to emerge unscathed as a Force Field (see page 166) with a Protection Rating of 35. Since this is purely due to the chance of the hit glancing off the armour’s strongest surfaces , there is no possibility of Overload.

Test the field first. If ti gets through test the armor. Yes Terminators are badass.

On a side note as a GM i'm considering allowing snipers to bypass the termie armor defelction capability by making called shots on the most vulnerable points (thinnest armor at the joints, et cetera).