Example of how it most likely doesn't work:
You have 100 control resist
Pet 1 give 50% boost (50)
Pet 2 gives 25% boost (37.5 - i.e. 25% of 150)
Total: 187.5 control resist
Example of how it likely works:
You have 100 control resist
Pet 1 gives 50% boost (50)
Pet 2 gives 25% boost (25 - 25% of 100)
Total: 175
All bonus stack. Even the same type companion bonus.
>>Example of how it likely works:
You have 100 control resist
Pet 1 gives 50% boost (50)
Pet 2 gives 25% boost (25 - 25% of 100)
Total: 175<<
This one is the right showing. The 175% control resist mean then control duration/1,75 = new control duration.
Stats increases are every time only additive, not multiplicative.
Comments
Example of how it most likely doesn't work:
You have 100 control resist
Pet 1 give 50% boost (50)
Pet 2 gives 25% boost (37.5 - i.e. 25% of 150)
Total: 187.5 control resist
Example of how it likely works:
You have 100 control resist
Pet 1 gives 50% boost (50)
Pet 2 gives 25% boost (25 - 25% of 100)
Total: 175
>>Example of how it likely works:
You have 100 control resist
Pet 1 gives 50% boost (50)
Pet 2 gives 25% boost (25 - 25% of 100)
Total: 175<<
This one is the right showing. The 175% control resist mean then control duration/1,75 = new control duration.
Stats increases are every time only additive, not multiplicative.
Duration = Base duration / (1 + percentage stat).
For example, if a CC normally lasts for 5 seconds its new duration under a 50% Control Resist would be: 5 / (1 + 50%) = 3.33 seconds.
Hence, I expect, it is an additive stacking with that formula, e.g. from above: 5 / (1 + 50% + 25%) = 2.86 seconds.