Good question! Cartel Market Certificates actually break our normal rules that our items need to follow in order to be considered "valid" by development tools --they're a special exception. Certainly there is a lot of desire to for this change to happen based on the feedback in this thread, I will make sure the dev team is made aware of it. Keep in mind that although this is a possible change, it would not be a simple change to make and it can be a risky change since the item is already live!
Good question because Cartel Certificates are bound to legacy and those are stackable.
Hope that makes sense!