The gather page has the information on Virulent Sliver:
http://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=207919.
"7/1/2013 Abilities that Slivers grant, as well as power/toughness boosts, are cumulative. However, for some abilities, like flying, having more than one instance of the ability doesn’t provide any additional benefit."
Your slivers get two instances of "poisonous 1" which will trigger separately and give two poison counters.
As far as Coastal Piracy, each enchantment will trigger and you will draw two cards.