Would using 2 progressions of evil and 2 heroic actions per turn destroy the balance of Shadows Over Camelot?
This would make the game much easier.  The ability to focus effort on quests is how the knights win.  Travel time relative to card draws would go down.

Quick example, 5 player game, you have fought cards 1-5, so you go to the Picts quest.  It starts as your turn:


Progression of Evil / You move to quest
PE / You play 1
8 other PEs from other players
PE / You play 2
PE / You play 3
8 other PEs from other players
PE / You play 4
PE / You play 5 -- Quest Completes


Total PEs possibly ending your quest in a loss: 22

In the original rules, it looks like this:


Progression of Evil / You move to quest
4 other PEs from other players
PE / You play 1
4 other PEs from other players
PE / You play 2
4 other PEs from other players
PE / You play 3
4 other PEs from other players
PE / You play 4
4 other PEs from other players
PE / You play 5 -- Quest Completes


Total PEs possibly ending your quest in a loss: 26

So, by focusing the effort, you've saving 4 progressions of evils (the other player's turns) on your quest.