I have a DL in Office 365, there are two owners on this DL and thats what I expect. I have the same DL shown in the OAB and GAL as seen through Outlook 2013 - there is only one of the owners on this DL. Any ideas?
My first stop would be to check the DL in OWA, as that should be a true representation of what is in on the server. If you see two owners there, it has likely just not propagated out ot the local outlook. If you show one owner, then it did not get set right on the back end, and you could try re-setting in the portal (or your AD if you are running AADC)