Poison weapon DOES stack. If you hit something twice, it gets twice as many blue numbers floating up out of its head. So I don't doubt poison skin and poison weapon work together alright.

If you have an item with Reflect Missiles on it, give that a try when you're wielding a poisoned weapon. Each arrow you bounce takes away a poison charge, and whoever is hit by the arrow (not necessarily the shooter, if melee monsters have moved into the way) will start dying. You can easily dance throuch the elite orc areas, pinging off missiles and racking up the XP without even laying a finger on your foes.

Sadly, though, poison isn't all that viable once you finish up with the orcs; undead are immune to it, and you fight mostly those later on. Just put 2-3 points in Poison Weapon and enjoy it while it lasts.

You will rue the day you opposed me! Begin your rueing! I'll just... sit over here... and watch.