Poison is evil because it's not "fair". It's something you can sneak into a victim's food or drink, and kill them without them even knowing they're under attack, and without them getting a chance to fight back or defend themselves.
Similarly, a poison on a weapon is considered an insidious attack, one that allows you to kill with a single blow, denying the opponent a "fair" fight.
That being said: I know I posted a list of alchemical items I researched from a number of sources, all TSR/WOTC in origin. At least one of them was intended to raise the DC of a poison, while another added to a poison's effects.
Because this is official WOTC stuff, it kind of answers the question about applying more than one poison.
As a DM though, I'd not allow it except through such additives specifically designed to be mixed that way. The reason? The surface of a dagger is x number of square inches, no more. The surface area is what holds the venom, and trying to add more venom doesn't increase the blade's capacity to hold it. Even specially made blades with venom grooves etched into them have a limit. The capillary action can only hold so much venom, and the amount they're designed to hold is one "dose", no more. By trying to add a different poison you end up with a diluted blend of the two. Perhaps different effects, but still only one "dose".
Could you have a blade designed with extra venom capacity? More grooves, perhaps a hollow blade that works like a hypodermic? Could you use hollow arrowheads designed to break open when you try to pull the arrow out?
Sure you could, with DM's approval. They're not in the game, but the DM can allow whatever they want. But such weapons would be as illegal to own as the poison itself, as they'd have no legitimate purpose whatsoever, and would be readily recognizable because of their unusual and highly specialized design.
As a general note regarding the use of poison at high/Epic levels: We found that once the party Cleric reached the point where he/she could cast Hero's Feast as a matter of course, poison use fell by the wayside, since everyone in the party is immune to poison. It simply stopped being a real part of the game.
What's amazing to me is how few DM's, when faced with that prospect, fail to have the bad guys start with a Dispel Magic to try and take down that effect.
Still, the old favorite was the Periapt of Proof Against Poison.