Billy: Begrimed Chains + Grisly Chains stacks how?

Both decrease survivor's repair time by 9%. Begrimed Chains also inflict Mangled for 120 seconds. It says on the wiki that these two stack, I'm wondering do they stack in effect or time?

In other words, if I use both and hit a survivor with the chainsaw, do they have 18% repair speed reduction for 120 seconds, or 9% reduction for 210 seconds?

Answers