The perks seem to stack in that you don't have two 25% chances at getting a rare resource every ten minutes if you have two gizmos with that perk equipped. I only received one resource every 10 mins with two gizmos. It might stack by increasing the chance to 50%, but I don't know how that would get tested.
i don't think the cooldown is 5 minutes. I just did exactly 20min of zammy and got 8 items (2 logs, 2 cavefish, 1 rocktail, 3 rune ores)
22.214.171.124 20:13, March 6, 2016 (UTC)
I was actually just wondering if they had updated it in the most recent perk update. I felt liek I was getting more resources than I should have, but I wasn't paying a whole lot of attention, so I didn't think much of it. I'll have to test it later and see.