Each arrow in Barrage and Volley counts as an attack for most game mechanics. I'm pretty sure only one of your arrows would poison.
Easy enough to test, just take volley and poison tip signet to isle of the nameless and use the trainging dummies to see if it spreads poison to more than one target.
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" Capital letters were always the best way of dealing with things you didn't have a good answer to."
No. Poison Tip Sig is pretty much worthless unless you like casting a skill every 6 seconds or something. Besides, if you're playing PvE, then poison for 14 seconds is virtually worthless because your team is usually roflstomping the mobs and by the time you poisoned them, they're dead. Unless, of course, you're countering a bosses' unconditional health regen.