A creature that is taking bleed damage takes the listed amount of damage at the beginning of its turn. Bleeding can be stopped by a DC 15 Heal check or through the application of any spell that cures hit point damage (even if the bleed is ability damage). Some bleed effects cause ability damage or even ability drain. Bleed effects do not stack with each other unless they deal different kinds of damage. When two or more bleed effects deal the same kind of damage, take the worse effect. In this case, ability drain is worse than ability damage.
If a creature has damage reduction from more than one source, the two forms of damage reduction do not stack. Instead, the creature gets the benefit of the best damage reduction in a given situation.
So DR5/magic and DR2/magic don't turn into DR7/magic, but DR10/silver and DR10/adamantine are usable together in that you can pick the one that's best for you at the time. It's like having resistance to energy (fire) and resistance to energy (cold), you can use both, they aren't subject to stacking because they aren't identical effects.
(Exception: if using the armor as DR alternate rule, that DR stacks with other DR.)
Best Answer
From the PRD (emphasis mine):