You'd need a huge free range. Apparently they are often less stressed when around mates, dont make such big clutches, and lay less often. Of course it depends on personalities and what not, plus you have to be a very experienced keeper to know the (often) very subtle signs of stress.
I would advise against it unless you have several years of experience, know both animals extremely well, and have a very large free range area (I wouldn't try it in less than an 8x8 free range with many basking spots, watering spots, feeding spots, and so many plants that they are extremely hard to find without digging around in the free range. That's my opinion, and what I would do if i ever housed two panthers anyways.)