How does that actually work? I had about 10 damage health potions (potency 21 dmg each) and placed them all on a Whiterun guard. He drank one and the rest stayed in his inventory. Do I have to place each one individually and wait to he drinks it to place another? Or am I doing something else wrong? Any help or explanation much appreciated.

They only drink one poison regardless of how many you reverse pickpocket onto them. From what I have been able to tell, you have to wait until the effects of the first poison wear off to do the same with a second.

Well that seems very tedious. Glad I saved right before I chose that perk! Thanks for your help.

Lingering poisons are best if you want to kill someone with the Poisoned perk. As are Fortify Alchemy enchants/perks.