With Macy’s and J.C. Penney back to battling over Martha Stewart in a Manhattan court, it’s easy to forget how much both retailers may suffer if one of them fails. The reason: both chains are under the same roof in some 400 malls nationwide. While that shared turf helps explain why Macy’s lawyer Michael Platt brought rival Martha-branded pitchers to court on April 9 — so much for exclusivity! — it also conveys an element of shared fate