Snake stance is passive poison damage and if you inflict twice before the DOT expires it doesn't stack.
You can get more info on that from here
https://docs.median-xl.com/doc/wiki/poisonBut a good example from that page is this:
Your snake stance does 100 poison damage per second over 5 seconds.
You have a pair of gloves that do 10 poison damage per second over 9 seconds. First you add the damage per second=100+10=110. Then you add and average the duration = (5+9):2 = 7. Now in order to calculate the total damage, all you need to do is 7*110=770. As you can see, the not so impressive gloves, increased our total poison damage by 54%.