Scortched, Question on how it works exactly.

I wrote about this as a minor issue in my other post, and have tried the practice tool on dummys to see. However, I'm too blind dumb or both to tell.

What I mean is does scortched work like slows, or does it stack with other dots. I ask because the tool tip and wiki didnt say last time i checked a week ago. But some champions have dot damage, ap champions in general if they buy get a dot damaging effect. Do the effects stack up too gether so you get max value out of the rune and item. Or does it simply default to the higher number and you lose out on the damage from the other. If a player could test this for me I'd be very greatful, sorry i'm an idoit and can't figure it out myself.