Multiple Different Poisons on One Weapon?

I was wondering how people would rule if you can apply multiple different poisons to one weapon or not. For example, if someone is playing a Grung Assassin Rogue with the Poisoner feat, can they use the Grung's Poison Skin feature, the Poison Cunning Strike option, and the Poisoner feat's Poison all on one attack?

There is nothing in the rules saying you can't combine those things.

There's limited utility in doing so, though, since applying the Poisoned condition to a creature who's already Poisoned doesn't do anything. But I guess it would give them more opportunities to fail the saving throw.

I don't see why not. It does make sense to do this from a Rogue's perspective too.
The only objection one might have is if they're thinking like a programmer and D&D like a video game, where the poisoning mechanic would be a state machine with a "poisonous" state for your weapon and the specific type of poison applied to it.
But D&D is not a video game :)

Quote from adaquet >>

I was wondering how people would rule if you can apply multiple different poisons to one weapon or not. For example, if someone is playing a Grung Assassin Rogue with the Poisoner feat, can they use the Grung's Poison Skin feature, the Poison Cunning Strike option, and the Poisoner feat's Poison all on one attack?

 Not exactly the same scenario, but I asked a similar thing in Can a weapon that provides bludgeon damage administer Poison. Some answers I got:

Quote from Plaguescarred >>

As for 5.5E rules specifically phohibiting it, i'm not sure there's any. Poison in the DMG doesn't address it and the core rules only has rules against Combining Spell Effects, which exclude Poison. 

Something in the rules a DM could use to shut this down could be on principles for Players Exploiting the Rules;

Rules Rely on Good-Faith Interpretation. The rules assume that everyone reading and interpreting the rules has the interests of the group’s fun at heart and is reading the rules in that light.

Outlining these principles can help hold players’ exploits at bay. If a player persistently tries to twist the rules of the game, have a conversation with that player outside the game and ask them to stop.

Quote from Wysperra >>

Might fall under the duplicate effects rule. Similar to multiple spells of same name. Or the ruling that most people make with multiple flasks of oil.

Quote from MaximusArael020 >>

Quote from TarodNet >>

Random thought. Not something I'd allow in my games, but Injury says:

Injury poison can be applied as a Bonus Action to a weapon, a piece of ammunition, or similar object.

RAW, could you apply, for example, the Serpent Venom poison multiple times to the same weapon? Or is there any rule I'm missing?

 Are you asking if you can apply multiple instances of the poison to one weapon for a single attack? Or applying it in different rounds?

I would say you cannot apply it, say, 3 times to a weapon and then attack for an extra 9d6 damage (assuming they fail 3 saving throws). It simply says "a creature subjected to the poison", so I would say the amount on the blade doesn't matter. It does the same damage if you dip the point of a throwing dart in the poison as it would if you coated an entire great sword blade in it.

But can you apply it round after round if you have enough of it? RAW it seems so. It doesn't provide a lasting effect, only extra damage. There's nothing in the description that suggests that you automatically succeed on saving throws against it or are immune to it after you've been subjected to it (something like the language from saving throws against certain charm effects from various monsters). If you have a supply of 5 uses of the poison then I would think you could apply them with your bonus action before each new attack and do 15d6 worth of poison damage if they fail all the saving throws. You're using your action economy resources, as well as disposable resources, to do it. 

I would assume in-game something like this might be hard to come by.

I do not think this is an ingame problem, but instead a real world physics problem. 

That is, applying a second poison would remove/undo the first poison.  

If you dipped a dagger in (illegal upper drug that makes your heart beat super fast), pulling it out with a thin coating of white dust on it, then attempted to dip the same dagger in a liquid paralytic poison, what do you think would happen?

Most likely the liquid would wash away the white powder. 

As a DM, I would rule that to apply a 2nd poison:

1) You need a poisoner's kit to even attempt to do this without removing the first poison.

2) You need to roll a tool use (Poisoner's Kit) to attempt this.  Under 14 = both poisons negated.  15-18 second poison applied but first removed.  19-22 both poisons apply but with a -4 penalty to the DC.  23+ = both poisons applied at full potentcy.