Title says it all. Can I apply two different poison-types to a single weapon or arrow?
It's possibly legal but also possibly not a lot of fun
Like this answer, so far I've found nothing specifically prohibiting the application of multiple doses of the same or different poisons to a single weapon or unit of ammunition.
However, this DM would not allow such a practice.
That's because it's trivial for NPCs and some PCs to acquire large amounts of poison, and, if a weapon can have multiple doses of poison applied to it, determining why folks don't coat their weapons in, like, 20 doses of drow poison (DMG 297) (75 gp; 0 lbs.) all the time becomes an exercise that's likely to strain the setting's verisimilitude. Seriously, many threats can be temporarily neutralized by—or eliminated outright by ability damage from—one failed saving throw against the right poison, and forcing an arbitrarily high number of saving throws against the right poison all but guarantees at least one failed saving throw.
Many players already resent the game's reliance on save-or-die effects, and a weapon coated in enough doses of poison becomes a save-and-die-anyway effect… at least, until combatants are immune to poison, which every PC and NPC in such a setting will be obligated to become. (Note that for combat purposes this can happen as early as level 3 when the 2nd-level Clr spell delay poison [conj] (PH 217) comes online, although PCs still must deal with the poisons in their systems when that spell's duration finally expires.)
This DM would rather not engage in the arms race that would ensue were he to rule that it's acceptable to apply multiple doses of poison to the same weapon. One possible save-or-die or save-or-suck poison effect per weapon attack is sufficient.