You use the best kind of DR for any specific situation.
e.g.
DR 5/magic and DR 5/silver.
When attacked by someone with a +1 sword, the DR 5/silver is used, and the damage is reduced by 5.
When attacked by someone with a masterwork silver sword, the DR 5/magic is used.
When attacked by someone with a +1 silver sword, you're up the creek.
I would imagine that this same rule applies to DR/--, and that if stoneskin is cast, then the character enjoys DR 10/adamantine. Unil attacked by someone with an adamantine weapon. At which point their DR becomes DR 3/--.