The chest on the left represents a regular private chest. The chest owner is able to add additional users on a second sign, to add an additional 3 players to that chest (6 in total). The chest on the right represents a trade shop representing the same objective, but an error message would appear while trying to attempt to add More Users. My suggestion is to be allowed to add additional players to tradeshop chests. This is a very useful addition for coop members, and something my coop members and I struggle with. If a shop or casino on the island goes out of stock, it would give access to the other coop member to restock that chest. It is also useful in keeping consistency in coop shops limiting the chest usage to 1 chest per item as all members would be able to access that chest.
I agree with this FULLY. My coop mate and I have a shop and have been trying to get items for it, it is definitely a hassle when trying to get blocks into the chest. (Since I already made the shops.)
This is something I personally struggle with with my coop members, especially casinos. It would be very nice to be able to take out the profits in the casino chest if it is full, and the same with tradeshop chests. Having to wait for the coop member to come online and all that extra stuff could be abolished from this suggestion.