So really I only get 8% extra damage and the other one don't proc, except the damage for monsters?
I use these with Kindred in the jungle
Max hp vs current hp.
If you have both, they are both applied on each auto.
So Unique passives with the same name, even if they are different items, don't stack. But if they are just labeled generic "Unique Passive" then as long as it's not the same item it will stack?
So the way it works is that "UNIQUE" passives do stack, but only if they're not from the same item. This is to prevent effects on item stacking up and becoming overbearing (imagine someone building 3 Spirit Visages and now they get 90% increased healing from all sources), while still allowing things like the 10% cooldown passives on items like Warmog's and Shurelya's to both work
"UNIQUE name" passives don't stack, no matter what item they're on. This is to prevent items like the Last Whisper line from being able to stack up their armor penetration.
So really I only get 8% extra damage and the other one don't proc, except the damage for monsters? I use these with Kindred in the jungle
are also affected by armor.
Ok so let me explain
Maximum HP damage is exactly that. Damage that is calculated based on your target's maximum health number.
If you hit a 4,500 hp Dr. Mundo and then hit a 3,500 HP Darius. Bloodrazor is going to deal a lot more damage to the Dr. Mundo due to how maximum hp scaling works.
Maximum HP damage will always calculate based off your target's total HP number regardless of if your target is at 100% hp or 10% hp when you fight them.
vs 4,500 HP 
: 4,500 x 0.04 = 180 on hit damage ** before armor**.
vs 3,500 HP 
: 3,500 x 0.04 = 140 on hit damage** before armor**.
Current HP damage is different.
Current HP damage is calculated based on the amount of health your target has at the time of the damage being dealt.
Current health damage gets WEAKER the lower HP your target is.
vs 4,500 HP at full HP: 4,500 x 0.08 = 360 on hit damage ** before armor**.
vs 4,500 HP at 50% HP: 2,250 x 0.08 = 180 on hit damage ** before armor**.
vs 4,500 HP at 10% HP: 450 x 0.08 = 36 on hit damage ** before armor**.
No they aren't. One literally does max hp damage meaning that it always is 4%. The other one does current hp damage which is a 8% max hp on first hit and decreases on each subsequent hit. If you are asking if they stack, then yes they do because they aren't the same and only exclude you from building another of the same item and doubling the passive.
They're not the same at all (even mathematically!), because you're taking down the target's HP by more than just the on-hit every auto. E.G., if your opponent has 1000 HP and your auto does 75+(item's AD)+(proc), then -- pretending resistances don't exist --
BorK would do 195 on the first auto, 179.4 on the second, 165 on the third, ~151 on the fourth...
Bloodrazor would do 115 damage on every auto.
Ofc, bloodrazor has more attack speed and is cheaper, but you get the basic idea on the math.
These are not the same. They do not share a UNIQUE Passive name. They are only UNIQUE passives so a champion can't stack multiple of them to get a ton of % health damage. Both will proc every attack.
So you will be dealing 8% current health and 4% maximum health every attack plus the rest of your attack's damage.
One is on maximum hp and one is on current hp.
