I don't think it's ever actually worth using both SotS and Mithridatism together when you could make Memory or a shimmer craft, I was just saying that even if your snake stance/amatoxin poison duration goes down because of the second item it's not necessarily a damage loss because of how good pierce is.
The pierce cap for maximum DPS is 135 for most farming, 145 in Scosglen, higher in labs (check the riftstones). 170 for most bosses up to 190-195 for labs bosses and 199 for Sam/Deimoss.
For maximum poison length, I don't know what the numbers actually are, but Ben- says it should be 175 poison pierce so I think it's going to be less than the other pierce cap on most enemies.