You need to redirect the activations of the dummy chests to a shared chest, which can be done via a script that you attach to the dummy chests:
ObjectReference Property kMasterChest Auto ;Fill this with the shared chest that is placed in the cellEvent OnInit() BlockActivation() ;Stops the chest, which this script is attached to, from opening its own inventoryEndEventEvent OnActivate(ObjectReference akActionRef) If(akActionRef == Game.GetPlayer()) ;Checks if the player is the one activating this chest kMasterChest.Activate(akActionRef) ;Opens the inventory of the shared chest EndIfEndEvent
It might be possible to do this without scripts, but I'm not entirely sure.