Hey guys, let’s go with this ok. Focus Master does not care whether the remaining incoming damage is still exact lethal damage. He also does not care whether there is any remaining incoming damage.
So if you have 2 focus masters, and one of your units is taking 3 damage, and that damage is exact lethal damage, both focus masters will lose 1 rune and your unit will take only 1 damage. If one of your units is taking 1 damage, and that damage is exact lethal damage, you will still lose 1 rune from each of your focus masters.