Getting Cross(-Edition)

Yesterday, I pondered the feasibility of a D&D party made up of characters from different editions. I also asked about it on The Hellsite Known As Twitter, and there Courtney Campbell of Hack & Slash replied with some actual experience of playing with cross-edition characters. Here's what Courtney said:

I've done this. It's fine. 5e characters and 4e characters require some hit point math, but it works pretty seamlessly. Turns out the player side and the DM side don't need the same rules to play, just an interface.

I've done this both in Numenhalla, and in home games with 2e, 5e, and 4e players. They can follow the PHB rules for their class. I just multiplied all damage 4e/5e characters took by 2, and cut their damage in half, and it worked fine.

I get the impression that Courtney is a top-level GM and perhaps it's not as easy as it seems, but it is possible!