Unless otherwise specified resistances to the same element do not stack, and you simply use the highest resistance to that element. This is a general rule that still applies in this situation.
Stacking refers to the act of adding together bonuses or penalties that apply to one particular check or statistic. Generally speaking, most bonuses of the same type do not stack. Instead, only the highest bonus applies. Most penalties do stack, meaning that their values are added together. Penalties and bonuses generally stack with one another, meaning that the penalties might negate or exceed part or all of the bonuses, and vice versa.
Note that although arguably the rules on bonuses do not exactly apply here, as resistance and damage reduction are not bonuses or penalties (depending on how you define 'checks or statistical scores'; arguably they are a bonus or penalty in the same way bonuses to damage are counted a bonus). However the rules for energy resistance specify that “Each resistance ability is defined by what energy type it resists and how many points of damage are resisted.” - thus the energy type could be considered synonymous with bonus type. I.e. fire resistance 5 stacks with cold resistance 10, just as an armour bonus of 1 stacks with a natural armour bonus of 2.
Similarly to the above sources the rules on Damage Reduction are very clear that multiple damage reductions do not stack, and give an example of how the general rule above applies to a situation they arguably don't explicitly cover.
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.