Check if a chest block is a double or single then get the inventory(s)

DoubleChestCheck

Block b = ....
BlockFace[] sides = new BlockFace[] {BlockFace.NORTH, BlockFace.EAST, BlockFace.SOUTH, BlockFace.WEST};

     for(BlockFace bf : sides) {

        if(b.getRelative(bf, 1).getType() == b.getType()) {

            Inventory left = ((DoubleChestInventory)((InventoryHolder)b.getState()).getInventory()).getLeftSide();
            Inventory right = ((DoubleChestInventory)((InventoryHolder)b.getState()).getInventory()).getRightSide();

        } else {

            Chest c = (Chest) b.getState();
            Inventory inv = c.getInventory();

        }
    }