Do the decoration multipliers stack?
Solution 1:
They stack, but having the Dodrio Tent and Mechanical Tauros won't grant you X3.5 grey elements drop, but X1.5X2 = X3 drops
Source : https://www.polygon.com/guides/2018/6/5/17429544/pokemon-quest-guide-spend-money and own experience with the game
Solution 2:
According to this site, they do stack! I had to dig a little to find it.