XDP simultaneous dual drive failure question

Hi everyone,

I read the XDP whitepaper and have a question about the dual drive failure scenario.

Because XDP is 23+2, it seems that we cannot have a dual drive failure simultaneously without data loss.

However, it goes on to say we can have no impact for up to 5 failed SSDs per xbrick so I am a little confused.

Does the dual drive failure mean it happens at the same exact moment? Or does it refer to a single SSD failing and the second one failing while the first is rebuilding ?

Also, what is meant by 5 failed SSDs per xbrick and no impact? This seems to contradict the statement that we cannot sustain dual drive failure simultaneously.