Two of my players are interested on investing on the Bleeding Critical feat.
I know that the bleeding from the feat stacks with itself.
Would the bleeding still stack the same way if each stack of bleeding came from a different PC?
Should I treat each bleeding as a different source of bleeding regarding to heal attempts and such or a single Heat attempt would stop both?
For this, I would say that since the feat specifically states that it stacks, and that specific rule beats general ruling, then it would stack no matter who did the attack. Also, rule of cool, if both player spend a feat and attack the same guy, and both crit, it would be lame to be denied their feat effect. Plus, if that enemy is being flanked by 2 rogue with bab +11, I really don't think the bleed damage is going to be his main concern. Depending on if they are Two-Weapon Fighting or not, that is from 6 to 12 attacks next turn, all with sneak.
For the healing part, neither the bleeding critical nor the Bleed general rule seems to mention that it does not stop all bleeding at once and that would be how I rule it in my games.
You usually use first aid to save a dying character. If a character has negative hit points and is losing hit points (at the rate of 1 per round, 1 per hour, or 1 per day), you can make him stable. A stable character regains no hit points but stops losing them. First aid also stops a character from losing hit points due to effects that cause bleed (see Conditions for rules on bleed damage).
Due to the plural I'm inclined to say that you would need only one Heal check to cure to of all the bleed effects, though I can see it the other way too.