Wit's End question

I only use Wit's End when playing as Skarner just now i'm adding it to my new prototype Fiora build

I know it's effect stacks up to a max of 5 times, increasing your MR to a total of +55 (30 base, 5 per stack, max 25)

but what about the reduction effect on your enemies is it a total of 5 stacks period, or is it per champion

example: if I hit an enemy 5 times and switch targets for whatever reason, does my new target also get their MR reduced by each hit, the result being both champions having the debuff stacked 5 times

or would the second target not get any MR reduction until the effect wears off of the first