There seems to be a minor catch in how this works. Yes, you get "Double Clovers" whenever you fill a product request in the co-op chat but you only get "Double Clovers" from the "Thank You" letters if you manually move them using the "Collect All" button.
I did a few experiments and if you already have 100 Letters, according to the old gameplay, any new letter is converted to a clover and placed in the HOL instead. Well, for Double Clovers an odd thing happens. This "101st" letter *and* the 100th letter move to the HOL (adding 2 clovers, appearing to be a 2x reward) however, the number of letters remaining in the Town Hall is now 99. In other words, it moved 2 letters into the HOL as clovers, not doubling the 101st letter. Did I explain that well enough?