While I'd like to explore the two separate units featuring Crosby, then Malkin idea... It's not going to happen.
As you guys said, they have no other D to approximate what Letang does. So that puts the second unit at a severe disadvantage in effectiveness.
While I think the PP with Crosby OR Malkin is better than the PP with Crosby AND Malkin; there aren't any tenable options to it.
The reason Crosby and Malkin together is worse, is that they are the same player trying to occupy the same spot and same role. Honestly, Malkin is better at it. But egos being what they are, you can't put Crosby on the 2nd unit as is.
Now, if you picked up a Gonchar (or type like him) specifically to log time on the PP point and help man a 2nd unit? Then maybe. But that's all hypothetical at this point.
As for the two-headed monster at ES? End of period only. Outside of that, I don't like it because you allow the other team to match their best D-pairing and checking line against your best line. As opposed to them being split, the other team can only match their best against ONE of yours. Which means you get Crosby or Malkin out there against the other teams second best defenders. And that is a WIN.