I'm running around with an assassin that was crowned with the dagger.
Since it's always poisoned, the appropriate message shows up on successful hits, where the monster in question doesn't have immunity.
However, when I obtained the poisoned hands corruption, another message started showing up, this one with with an excalamation mark:

I think the game should probably acknowledge that the PC is now using two sources of poison in combat and act accordingly, i.e. produce only one message, perhaps somewhat different, something like:
"The *foo* is severely poisoned! Sick green liquid seems to be oozing from its wounds!"
Additionally, I don't think the poison effects actually stack because I haven't noticed any increased frequency of "cringes in pain" messages and it took roughly the same amount of time for monsters to collapse.

Perhaps it's a good idea to somehow take double poisoning into consideration and increase the strength of poison attacks inflicted by the PC to some considerable level?
It would possibly make poison hands a situationally desirable corruption instead of something that players remove ASAP.