There's a ton of people you trade before you even think of trading the Penguins. At least, if you are a smart front office (which I'm not convinced the Penguins are...).
Crosby and Malkin, and their cap hits aren't the problem. This season, it should be noted that the problems have been magnified due to injuries to their 4 best puck moving defence. Letang, Matta, Erhoff, and Pouliot. They were stuck with a bunch of 'stay at home' defencemen that couldn't help with the transition game one bit (and it lead to hardly any good odd-man rushes, Crosby and Malkin never getting the puck with speed or space, and goals had to all be ugly like the one they scored last night....it basically completely neutered the skill that the Penguins did have).
But even with those defensemen healthy, the Penguins have some pretty big holes in their lineup, mainly their wing and have bad contracts. Neither Malkin or Crosby's contract are bad ones. They are actually very reasonable. The Penguins cap issues has more to do with paying Kunitz, Dupuis, Scuderi $3.5 million or more each/season on 4 + year deals. It has to do with the Penguins not focusing on drafting young forwards.
Before you even think of getting rid of Malkin, you try to get rid of the actual dead weight on the roster, and use that freed up space to try to build around your superstars, something the Penguins have failed to do throughout Crosby's and Malkin's tenure in Pittsburgh. I expect the Cap to keep rising, meaning the Cap hits of Crosby and Malkin keep getting more affordable, while depth players will be getting more and more expensive. There's just not a trade that will be worth it for Pittsburgh, unless Malkin himself really wants out.