I tryed to create a sell crate in my playershop for dirt, and the sign is sideways in front of the crate. Also my 2 trapped chests won't go together, and they are placed side by side.
I think that is only for trapped chests. Or it might be you can only use trapped chests to make doubles. I made a bunch of double chests after updating to 1.13.1.....
I'm not sure about the sell signs, but to create a double chest, shift while placing it otherwise it'll place as a single. [works for normal and trapped chests] **Update** Auto signs are disabled. You can either make a custom sign or place a blank sign then right click to show default data.