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.)