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Yes, presenting the reasons you should keep it on one domain [duplication of effort between two sites, splitting up potential links, duplicate content on the sites, possibility of Google thinking you are creating a link scheme by linking between the sites] might convince the decision-makers. If there is no way to convince them, for the magazine site I'd optimize for long-tail search terms relating to "magazine" or "travel" or whatever is unique to the publication's content. [And from reading the media kit, it sounds like luxury lifestyle content is an intended focus, so I'd probably emphasize that.]