Whenever you use Soul Harvest with 6pc JH it deals damage equal to 30 seconds of the dot - whether or not you have Quetz equipped.
What Quetz does, is make your dots tick twice as fast (when you dot a target, you'll see the damage be twice as big (instead of the damage shown twice the amount)), so when you SH with Quetz up, you're - in reality - dealing damage equal to 1 minute of the dot, whereas if you didn't have Quetz equipped, you'd only deal damage equal to 30 seconds of the dot.
All in all: Quetz doubles your DoT damage and the 6pc JH effect. My recommendation is to get Gloves, Shoulders, Chest, Pants and Boots. If you, like me, don't have the boots yet, but have the helm, you can just run around with the Quetz on, and whenever you want to SH a pack for example, equip the JH helm just before doing so.
I hope this clarifies some things, if not; feel free to ask!
When you use Soul Harvest with the 6-piece Jade Harvester's set, it only deals damage equal to 30 seconds of the dot, NOT 60 seconds if you have Quetzalcoatl equipped. If you do not have Quetzalcoatl equipped it deals damage equal to 1 minute of the dot.