Do its bonus stack with Aid Another's not-circumstancial bonus ?

Yes. They are bonuses of different types, and therefore stack. Untyped bonuses always stack with all sorts of typed bonuses, and also with most other sorts of untyped bonuses (the exception being that a bonus will not usually stack with itself if you would be entitled to it more than once, even if the bonus is untyped; this is known as the Same Source limitation). They could have made it an exception if they wanted by explicitly saying it does not stack with Aid Another, but that would be pointless. Thus someone using Aid Another while having the feat effectively doubles the bonus that they offer.

What does mean "your assistance doubles the gp value of items that can be crafted each day” ? Do you craft twice faster (thanks to a very twister way of reading "double the gp value you can craft each day") ?

That’s precisely what it means. Actually, you craft a little more than twice faster because you are hitting a higher DC thanks to the feat’s +2 circumstance bonus, but that particular line doubles your speed compared to if that line was not there. Your progress on any crafted item is measured in silver; when the silver value of your work crafting the item is equal to the value of the item in silver, you are done. If your crafting checks produce twice as much silvers’ worth of progress, then you will meet the item’s value in half the time.

Why is Cooperative Crafting a feat?

A good question; to my mind it’s not very good design. Aid Another was already sufficient, I think, to model someone helping you out. If anything, I would have made this sort of bonus contingent on hitting higher DCs with your Aid Another roll (since, as you say, DC 10 is remarkably easy to hit), and for having both people have the relevant feat. This seems like an unnecessary feat tax to me, even if the effect is reasonably good.

On the other hand, I’m not too familiar with Pathfinder’s options in this regard, but if there are enough effects out there that stack and reduce crafting times or crafting costs, that can lead to very overpowered characters if they can get them all (see 3.5’s Artificer). Which makes me leery of an effect that doubles crafting speed, particularly when you don’t need to take the feat yourself; a cohort could take it, you could hire an NPC who has it, or whatever. Alone, this feat is fairly week. In combination with other, similar feats, it might be overpowered. Which again would just lead me to wanting to have a lesser effect, achieved just by hitting higher DCs than 10 on Aid Another.

Creators usually don't roll to create magic items at all

Dungeons and Dragons 3.5 creatures usually don't need to make rolls to create magic items. A creature takes an item creation feat, picks an appropriate magic item to create, buys the raw material (usually costing half the price of the magic item), and, if the creature meets the prerequisites, spends the appropriate amount time, raw materials, and XP. When all that's done, the creature's created the magic item, and usually no rolls have been made. (There's more to this than that, of course, but these are the rules at their most basic.)

As an aside, when the magic item's creator purchases the raw materials for creating a magic item, the creator's usually purchasing everything (except the workspace) needed to create that magic item (except in specific cases). For example, a creator needn't itself use the skill Craft (leatherworking) to create by hand the actual masterwork leather haversack that's going to become a Heward's handy haversack. That masterwork leather haversack's cost is subsumed in the raw materials' cost. A DM may give a discount on the cost of raw materials needed to create a magic item if the creator makes or already possesses some of the raw materials, but that's usually a savings so small it's a debate not worth time at the table.

An artificer can ignore some of a magic item's creation prerequisites by making successful Use Magic Device skill checks

The typical artificer only needs to make Use Magic Device skill checks to ignore one or more of a magic item's prerequisite (DC 20 + the item's caster level). During the time the artificer takes to create the magic item, the artificer must succeed on a Use Magic Device check only once per prerequisite that the artificer doesn't meet.

Examples

• A typical artificer with the feat Scribe Scroll but without access to the spell cure light wounds must succeed on one Use Magic Device skill check (DC 21) to create a scroll of cure light wounds (1st-level spell at caster level 1) (25 gp; 0 lbs.). Such a scroll takes 1 day to create, expends raw materials valued at 12 gp 5 sp, and causes the artificer to expend 1 XP.
• A typical artificer with the feat Craft Wondrous Item but without access to the spell Leomund's secret chest must succeed on one Use Magic Device skill check (DC 29) to create a Heward's handy haversack (2,000 gp; 5 lbs.). Such a haversack takes 2 days to create, expends raw materials valued at 1,000 gp, and causes the artificer to expend 80 XP.

This is explained in greater detail below.

Long Explanation

The artifcer's extraordinary ability item creation says

An artificer can create a magic item even if he does not have access to the spells that are prerequisites for the item. The artificer must make a successful Use Magic Device check (DC 20 + caster level) to emulate each spell normally required to create the item.

Thus, to make a 1st-level wand of magic missile, an artificer would need a Use Magic Device check result of 21 or higher. To create a bottle of air (caster level 7th), he would need a check result of 27 or higher to emulate the water breathing prerequisite. (Eberron Campaign Setting 32)

The game wants you to use either a magic item's listed caster level (for magic armor special abilities, magic weapon special abilities, and wondrous items) or an item's appropriate caster level (for potions, scrolls, staffs, and wands). This is why the example bottle of air—the typical one having a caster level of 7—requires a Use Magic Device skill check (DC 27).

(Note that this is the path of least resistance. Technically, because—like many magic items—the bottle of air's caster level is not a prerequisite for its manufacture and the spell water breathing is a 3rd-level Sor/Wiz spell, an artificer could make a bottle of air with a caster level as low as 5, but the artificer is powerful enough without reducing to their absolute minimums the caster levels of printed items.)

The artificer must make a successful check for each prerequisite for each item he makes. If he fails a check, he can try again each day until the item is complete (see Creating Magic Items, page 282 of the Dungeon Master’s Guide). If he comes to the end of the crafting time and he has still not successfully emulated one of the powers, he can make one final check—his last-ditch effort, even if he has already made a check that day. If that check also fails, then the creation process fails and the time, money, and XP expended to craft the item are lost. (Ibid.)

Each prerequisite the artificer doesn't meet requires the artificer to succeed on a Use Magic Device skill check (DC 20 + the item's caster level) once and only once during the item's creation. If the item takes more than 1 day to create, the artificer can retry a failed check each day of the item's creation. If the artificer's about to complete the magic item and hasn't succeeded on one or more Use Magic Device skill checks to fake a prerequisite, the artificer gets one last chance to make a Use Magic Device skill check for each unmet prerequisite on which he hasn't succeeded. Failure on any of these checks wastes the gp, time, and XP. Success on all of them creates the item.

For purposes of meeting item prerequisites, an artificer’s effective caster level equals his artificer level +2. If the item duplicates a spell effect, however, it uses the artificer’s actual level as its caster level. Costs are always determined using the item’s minimum caster level or the artificer’s actual level (if it is higher). Thus, a 3rd-level artificer can make a scroll of fireball, since the minimum caster level for fireball is 5th. He pays the normal cost for making such a scroll with a caster level of 5th: 5 × 3 × 12.5 = 187 gp and 5 sp, plus 15 XP. But the scroll’s actual caster level is only 3rd, and it produces a weak fireball that deals only 3d6 points of damage. (Ibid.)

(That is, it's a weak fireball unless a level 3 artificer needs to explode the evil baron's squad of 10 War1 guards from 500 ft. away or something.) The artificer, then, is allowed to create items early and is tacitly granted potential access to magic spells (via items that emulate spells) before the party wizard.

An artificer can also make Use Magic Device checks to emulate nonspell requirements, including alignment and race, using the normal DCs for the skill. He cannot emulate skill or feat requirements, however, including item creation feat prerequisites. He must meet the caster level prerequisite, including the minimum level to cast a spell he stores in a potion, wand, or scroll. (Ibid.)

This allows the artificer to make items like a dwarven thrower (with its prerequisite of creator must be a dwarf of at least 10th level) or a weapon with the magic weapon special ability unholy (with its prerequisite of creator must be evil), but an artificer can't employ the extraordinary ability item creation to make, for example, a folding boat (with its prerequisites of 2 ranks in the Craft (shipmaking) skill).

Note: The answer above assumes the DM rules that the artificer's special ability item creation can be used concurrently with standard item creation instead of replacing standard item creation. If the DM rules the artificer's special ability item creation replaces standard item creation, the artificer will need to make successful Use Magic Device skill checks for all of a magic item's prerequisites, even those the artificer already meets. This DM does not endorse such a ruling.