wits end

From reading it, it does sound like it would cap out at 5 stacks of MR lost on the champion hit. The benefit would be that if you are both hitting him, you'll apply stacks faster.

Any debuff that can be marked from the same source will be limited by the same rules.

Since WIt's End is the source of this MR Shred, it will cap at 5 stacks, regardless of the number of champions using it. Same for Black Cleaver.

It would only stack to the cap. The only difference is you would apply it faster since two of you are attacking with it.