Does Seviper's "More Poison" Ability stack if I have more than one of them in play?
Yes, each of your Pokemon with a "More Poison" ability will add an additional damage counter to your opponent's Poisoned Pokemon in between turns.
Source: Burning Shadows FAQ (2017-07-20), TPCi Rules Team (2017-08-03)

Relevant cards

Note: All applicable cards may not be listed below.
SM3_50
SMA_SV15
SMA_SV15